I've got an unfinished ES-1 and I have tried full-sized pots in the F-hole. You can fit a single one in at an angle, but I haven't tried it with full-sized pots and the full wiring harness. I prefer to use stiffer wire (vintage-style cloth-covered push-pack) than that supplied with the kit, which adds an extra level of complexity when trying to fit it all through the F-hole. Mini pots make it all so much easier. And if you have to force a part through the hold, you're more likely to break a solder connection, so that it all has to come out again to be fixed. I much prefer full-sized pots, but it will be mini-CTS pots used for the ES-1.

As you say, it never hurts to have spare pots handy.