If you're spending the money to add a panel anyway, you might as well reduce the changes of having to expand it later. Unless you're installing the second panel really far from the main, you're likely not going to spend much more to put in the 100 amp panel over a 60 amp panel. Oversizing the panel will not hurt anything but your wallet, so why not do it?
The installation will be exactly the same either way, the only differences will be the size of the breaker in the main panel, the main breaker in the new panel, and the size of the conductors between the panels.
You can read this answer for more detail, but likely you'll need four 3 AWG copper conductors to feed a 100 ampere panel. Of course the size will change based on the length of the run, and other factors. So make sure you verify the size once you know exactly how long the wires will be, and how they'll be run.
Did a quick search for wire prices, and it looks like it's about $0.20 per foot difference between a 6 AWG stranded copper wire and a 4 AWG stranded copper wire. Then another $0.20 per foot difference between 4 AWG stranded copper wire and 3 AWG stranded copper wire. So you'd be looking at an $0.80 per foot difference between 6 and 4 AWG feeders, and an $0.80 per foot difference between 4 and 3 AWG feeders.
Have you tried searching the site? The panel feeder size question has been answered many times. Here's one such example.
Is you're planning to pull a cable, you'll use 3-3-3-5 USE cable with copper conductors (1-1-1-3 for aluminum). If you're pulling individual wires, you'll want four 3 AWG THWN wires (hot, hot, neutral, ground)(black, red, white, green) (1 AWG for aluminum). If the run is going to be really long, you'll want to calculate the voltage drop and adjust the conductor size accordingly.
If you use the 3-3-3-5 USE cable, you don't need conduit. If you want to install it in conduit, you'll need 1 1/4" schedule 40 PVC. If you use individual THWN wires, you'll also use 1 1/4" schedule 40 PVC.
You'll have to keep the grounded (neutral), and grounding bars separate in the secondary panel.
You'll have to install a grounding electrode system at the garage, and bond the equipment and grounding conductor to it.
NOTES: This is a very broad, and generalized overview, which lacks specific details for your particular situation. You may need the advice of a licensed Electrician, to provide a more detailed plan based on the actual job site.
Best Answer
Long-distance circuits are a huge opportunity to spend way too much on wire unnecessarily. Given the amount of money that is at stake here, let's debunk the usual 3% knee-jerk.
There are two huge mistakes when made when sizing circuits.
Voltage drop is proportional to current right now
Remember that voltage drop in a circuit is a function of current actually flowed. Here's what's not true: "The voltage drop the calculator says will always apply to all loads". Actually it will never apply to any loads.
Suppose someone puts in monster wire on their "50A" circuit and gets to 2.5% drop at 50A.
You draw 10A @ 120V. You actually get 1.0% voltage drop. Not what we expected, eh?
What happens if you have a "30A" dryer, which actually is 23A, and that actually is about 21A on the 230V side and 2A on the 120V side. So drops of 0.525%, 0.05%% and 0.575% per leg. The 240V heating element sees 1.1% voltage drop, and the 120V mechanism sees a 0.575% voltage drop.
A 40A (9600W) heater sees 2.0% voltage drop, which it doesn't need -- heaters will work on 30-40% voltage drop.
Getting the idea how wasteful this is?
For a welder, consult with your manufacturer, but voltage drop is pretty normal for a variety of reasons inside welders, so it's probably not going to bother them all that much.
Transformers
Then there's using transformers to step up voltage. All fixed-installation wiring is rated for 600V, and there is nothing wrong with stepping up power that high for the transition. Transformers are expensive, but on a long haul, they're cheaper than wire. Often, simply "stepping up" the circuit to 240V, and using a transformer at the far end to make 120V, is all you need. I have plenty of postings about this.
Let's run some numbers in your case.
And to be clear, we'll be running aluminum wire, because running so much copper isn't even stupid. Use the goop and torque to spec.
Scenario 1: Obedient Consumer.
Let's do exactly what the voltage drop calculator and wire companies say we should do. Compute on breaker trip and stick to 3%. 3/0 wire would have 3.05% drop and the computer says "no" to that and computers are smarter than us, so we are forced to 4/0 wire. 750' of 4-wire 4/0 URD will cost $2145, trench it at 24" and we're done.
Scenario 2: Compromises.
On a 50A breaker, actual draw shouldn't be more than 80% or 40A. We go for 4% voltage drop at 40A. This comes up as 1/0 wire. 750' of 4-wire 1/0 URD will cost $1297.
Scenario 3: 480V Transformers.
In this case we use 15 KVA transformers and breaker for 60A (which is more power). The transformer halves the current, which also halves the voltage drop, and voltage drop is only half as important anyway, since it's coming off 480V instead of 240V. So now we can happily use much smaller wire. 240V at 40A becomes 480V@20A. With #4 wire, voltage drop @ 20A draw happens at 2.45%. Even if we max out the circuit to 60A@240V (30A@240V), voltage drop is only 3.67%. Further, we only need 2 wires between the transformers, because neutral is created locally by the transformer.
Scenario 4: Mini-transformer.
In this case, we only aspire to provision 20A (16A) practical, but we use a (smaller) transformer so we can transmit at 240V on 2-wire, even though we are only using 120V. Fortunately, these smaller 5 KVA transformers are readily available on Craigslist for about $100. If we go for only one 20A circuit, then we base voltage drop on 8A@240V and the calc says we're just fine with 6 AWG aluminum at 3.58% drop.
However, instead, let's go for two 120V circuits at 20A breaker (that's table saw and dust collector). That will realistically be about 24A together, or about 12A@240V, but let's assume circuit continuous max of 16A x2, so 16A@240V. Voltage drop calc says that happens at 4.57% with 4 AWG Al.
And hey, that's the same #4 wire we use on our 480 scenario, and we know we can pump that to 60A or even 80A with better transformers.