- If your target succeeds on the Ref save, the sphere doesn't form in the first place: "Reflex negates", thus the effect (1-foot-diameter/level sphere centered around a creature) cannot be triggered, thus the sphere cannot form.
- Probably, and probably. The spell description is a bit vague, but it seems entirely reasonable that others could be trapped within the sphere (the description suggests this is the case: "The sphere cannot be moved ... by the struggles of those within.") and that those others would get a ref save to dive out of the way.
I advise conferring with the GM about how they would adjudicate such a sphere catching multiple creatures before spending too many resources on it (which may require casting the spell in a mock combat situation with your party).
If you're the GM, decide how it works and be consistent. It's fine if other spells (or other force-effects, even Forcecage) work differently, but make sure that Resilient Sphere always works the same (barring specific situational modifiers, of course).
Were I GMing, I would give non-targets who would be caught a ref save to dive out of the way, in the direction of their choosing (using up movement from their next action); an acrobatics check (or saving by 5+/hitting a nat-20 on the check) would let them end the movement standing, else they'd be prone. Large (or larger) creatures who are partially in the space would get the ref save to choose whether to end up inside or outside of the sphere; failure would put them on whichever side the caster wants them to be on.
Squeezing
what if you arrange your panels so there's a section only 2" wide? Is the creature squeezed between the walls?
First, the space a creature takes up is not the same as the size the creature controls.
A creature's space is the area in feet that it effectively
controls in combat, not an expression of its physical
dimensions. A typical Medium creature isn't 5 feet
wide, for example, but it does control a space that
wide. If a Medium hobgoblin stands in a 5-foot-wide
doorway, other creatures can't get through unless the
hobgoblin lets them. (PHB 192)
The rules on squeezing are pretty clear for most cases:
A creature can squeeze through a space that is large
enough for a creature one size smaller than il. Thus, a
Large creature can squeeze through a passage that's
only 5 feet wide. While squeezing through a space,
a creature must spend 1 extra foot for every foot it
moves there, and it has disadvantage on attack rolls
and Dexterity saving throws. Attack rolls against the
creature have advantage while it's in the smaller space. (PHB 192)
So, a medium creature can squeeze into a space that is Small, which according to DMG page 6 is a 2.5x2.5 ft square it controls.
Squishing
2 feet is less than 2.5 feet so what happens? The rules simply don't say, and a ruling is necessary. So here are some options the DM might rule.
The Creature Takes Bludgeoning Damage
The DM could rule that like in real life when getting stuck in a space that's too small, the creature gets squished into the space, all the things of being squeezed applied, and take some amount of bludgeoning damage. They may also say the space so cramped he's not just losing a foot of movement, but maybe he's in difficult terrain or even restrained.
The Creature is Pushed The Other Way
The DM could rule there isn't enough room and spell isn't a damage spell, and the spell simply shunts them the other way instead.
The Creature is Squeezed
The DM could rule that sizes are an approximation and abstraction. All medium creatures fit inside of a 5' square, but maybe the elf has a slight enough frame can squeeze not only into a 2.5' square but even into a 2' square. The DM could continue that logic to even smaller spaces if they felt appropriate.
Best Answer
Unfortunately, you can't catch 'em all.
While you may be able to encapsulate smaller monsters, the Targets section of the spellcasting rules (PHB p. 204, BR p. 102) state
Since you target a point - you must be able to see that point, and as framed, you cannot currently see inside the Tarrasque.
But let's say you could, for whatever reason. The paragraph continues with,
So while Wall of Force allows the caster to decide whether or not to shunt the creature forward, it ensures that the dome comes into play on the outside of the Tarrasque.
Seamus asked, "Wait... Can't you target a point near the Tarrasque that you can see, but still creates a sphere that intersects the Tarrasque?"
The rules for Area of Effect can be found in the next section of the rules. The rules state,