A 49-key will cover 90+% of what you’d want to do in the first year or two of traditional training. A 25-key will get annoying sooner, but if you know space is going to be a real issue, it’s just a percentage question again. You could just decide your skill will be “25-key keyboard” and adapt the training material. But I’d personally consider 49 the realistic minimum to not regularly feel claustrophobic.