If your intention is to control Large and larger undead: that's pretty easy.

If your intention is to create Large and larger undead: you will either have to wait for WotC to produce the appropriate material, or... get creative with your DM's world.

A wizard of the school of necromancy is given little restriction on their use of their Command Undead feature other than the undead's Charisma and Intelligence (Player's Handbook, pg. 119):

Starting at 14th level, you can use magic to bring undead under your control, even those created by other wizards. As an action, you can choose one undead that you can see within 60 feet of you. That creature must make a Charisma saving throw against your wizard spell save DC.

...

If the target has an Intelligence of 8 or higher, it has advantage on the saving throw. If it fails the saving throw and has an Intelligence of 12 or higher, it can repeat the saving throw at the end of every hour until it succeeds and breaks free.

Therefore, any undead you can see can be yours, as long as it is dumb and ugly enough and you are powerful enough.

Unfortunately, there are no spells or abilities published yet that allow creation of giant undead. However, if they exist in the world you inhabit, they must come from somewhere, right?

Therefore, your first task would be to find out what makes giant undead creatures in the world. Find out what is creating them and where:

• Is it an NPC necromancer who stumbled across an ancient artifact that can amplify the effects of necrotic spells (as in my campaign)?
• Is there a haunted graveyard where the restless dead can't help but stretch their legs? (common trope)
• Or is there a particularly unholy bog somewhere nearby that just imbues corpses in it with undeath?

Once you find that answer, simply harness that energy, adapt it to meet your needs, and you will be churning out giant undead slaves in no time.

Calculate CR for the Necromancer and undead separately then use encounter adjusted experience for the group

In 5e CR is used to determine if a creature is appropriate with characters of a given level based on its damage/round and defenses. Creatures with a CR above the PCs level risk overwhelming them either by killing them instantly or soaking up damage beyond what the PCs resources can dish out.

While a Necromancer with a bunch of undead is going to do more damage than a Necromancer alone it isn't going to do this damage as a couple huge attacks like one high CR monster would. Similarly as the Necromancer's group loses HP it will lose creatures unlike a high CR monster suffering critical existence failure. Due to these differences the right way to stat this is by determining the CR of the necromancer and then determining the difficulty of an encounter with them and the group of undead.

Remember the undead will be available when calculating the Necromancer's CR

My experience has been that determining offensive CR for a spellcaster is tricky as a spell's effect often varies with how many allies they have. For example your Necromancer could cast Bless which would increase their groups expected damage by a hard to calculate amount. In your case since you know there will be 4 skeleton/zombies you can get a more accurate offensive CR by including their presence in your calculations when determining how effective the Necromancer's spells will be.