A smidge is a very small amount. Just a smidge, a smidge more salt, a smidge too far.
It is informal and affectionate, and it belongs to cooking and to adjusting things by hand. Smidgen is the longer form and means the same. Neither is a measure of anything, which is the point — a smidge is as much as your judgement says.
1nounsmidgeA very small amount.
“Could I have just a smidge more cream on my cake?”
smidge is a small amount.
Also accepted in games: modicum