Spend time in modulargrid and youtube and design cases at small sizes. Try 54HP, 84HP, 104HP, before you go any larger.
If you design a case that you think will work, try redesigning it with different modules so that similar functionality will fit in fewer HP.
This process teaches you how to achieve density and cut costs. Now switch gears and think about ergonomics, possibly spread out a little bit.
You want oscillators, wave manipulators (folders, filters), VCAs, attenuators, envelopes, modulators, and sequencers. It's not impossible to fit all of that in a lunchbox case. If you already use a computer for music, consider an Expert Sleepers ES-8, so that you can use the computer to stand in for modules you don't own yet.
Or, like, don't do any of that. It's expensive, and the end result is either noise, sound, or music, none of which are things society is bending over backwards to give you money for.
You can do it all in Pd for free. You might be fighting ergonomics a lot of the time, but some MIDI controllers will help, and they're a lot cheaper than eurorack.