Hi Funbun,
Toodee's comments are spot on. You actually can't go too big if it comes to the rack (but you can go too small easily). However I know from one of your previous posts that you are looking into minimal music (or how you call that exactly?) so you might get "away with it" ;-) with a medium rack. Advisable is at least 3 rows (3 x 3U = 9U) and since you got already those 104 HP rails, 3 x 104 = 312 HP, it might just work out for you but on the bit longer future it might be still too small. So if you have the space and enough wood :-) Perhaps you could consider a 4 or 5 x 3 U case, as I mentioned in the other post, you don't have to fill them up straight away, take your time to get experience with modular and slowly try out more modules.
Good luck and kind regards, Garfield.
For review reports of Eurorack modules, please refer to https://garfieldmodular.net/ for PDF formatted downloads