Small spaces can become more confining after being there for a while, that’s why I would suggest nothing smaller than a 4x4, a foot will make a difference. Of course it also depends on how large you are, at 6’2’ and well over 200 a 3x4 wouldn’t fit me.
I laughed at my son, he is building a 4x8 and said eventually he will build a 8x8, His justification: Big enough for two and provide enough room to sleep in if he wants to spend the night.