Hmmm, we disagree about this
Using a waveboard (which is what I do) for the hydrofoil, gives IMO no similarity whatsoever to when riding this wave/surfboard with surf fins instead.
Mounting a hydrofoil on a waveboard, is not just like using much bigger fins...
It totally changes everything, and the board can not be ridden on the surface in a way that resembles what used to when it was a waveboard.
You can not edge the board, for one, and the way it ride over waves has no similarity either IMO, because of the wing.
So no balance comparison, and you can not use your exprience from an existing board, if you mount a foil wing.
Also, mounting inserts for a plate (that usually require another plate and more reinforcement in the board), or for a tuttle box - will ruin the board (slightly or a lot) as it will not have the right tail flex anymore.
Maybe a luxury problem and only apparent if you ride waves a lot on your non-foil waveboards (like I do), so you become sesitive and want the best with no compromises.
Will just note this as an issue though.
I've filled the finboxes up with silicone on the waveboards I've changed to kitehydrofoilboards, as I will never use them for both.
But as said, I am picky, of course it will work okay having a waveboard for both waveriding and hydrofoiling.
The big race or racelike boards, will make learning SO much easier and faster.
You can just ride with the board on the surface, and slowly but safely pop the foil up for short intervals, while learning your body to adapt to pitch and control and to forget about edge pressure.
This becomes much more difficult with a small waveboard with a foil under, especially in small chop waves.
Later, after initial learning, when you can ride okay, it will be much easier to both tack and jibe on a racelike board, because of its stability when on the surface (maybe an advantage the first years as you will still be learning most likely).
The mast, when board on the surface, works like an anchor in a way, that makes the board+foil work completely different as used to on waveboards and raceboards, although resembling somewhat more to big finned raceboards, indeed.
Big boards will just be easier to learn to ride, to learn to turn - no doubt
When you have learned and are medium experienced, you can just ride a really small flat plate (and maybe you can, maybe you can not, tack and jibe).
You might be in the water much more - but hey, also fun
Only when you can nail flying tacks and jibes almost every time, I will say the board does not matter
Having said above, of course you can easily use a small waveboard for learning, it will still come quite fast, only downside is you will probably have to use much longer for learning non-foiling tacks and jibes.
Anyway, just my view - it seems there are different opinions about this.
Not regarding size though, because pitbulldoug also writes you will want a really wide/big surfboard