Fingers. He looked like my hand because I was a silly goober who thought imaginary friends were dumb, but had no issue with anthropomorphizing my own hand.

Kiragushya a 2 1/2 foot tall dancing sword waving samurai

Weirdly, now I think about it, they were just me gender swapped

I never had one as a kid either, not sure if I could have with being on the aphantasia spectrum. I do, however, have a lot of characters I've roleplayed with, and they all go by what one might say is a set system I use.

A blue Pikmin named Lure who would follow me around everywhere hahaha

My imaginary friend was very tiny, and I'd imagine him cartoonishly jumping around whatever environment I was in so he could talk to me on my level. I used to hide out in the bathroom a lot, so oftentimes he'd be jumping on top of the toilet paper roll, then scampering up to the counter so we could have a proper conversation. If I was in the car, he'd sit on the edge of the window.

The best way I could describe him looking back would be like the vault boy from fallout. Not the same clothes or look, but just that cartoony style. I probably imagined him to be a little darker and with different colored hair.