I mainly eyeball mine too, that or fold it on the diagonal until it makes a triangle, so they are usually about 7 inches square. The Ballband cloth I just finished is much larger than that and not square, caused it was turning into a towel.
The only difference between an experienced knitter and new knitter is that the experienced knitter makes bigger mistakes faster. Be bold; there are no terrible consequences in knitting.
OTN: Bad Math scarf, Blocks in Blocks Afgan, pet snuggles for charity, and as always, dish cloths.