Comparative Sizes

I’m presently painting a portrait. I don’t have a complete photo because I’m making various parts up from various images. At the moment the hands are too big.

I’ve noticed that for me the farther apart the items are on the painting the harder it is to gauge their size. The best idea I’ve read so far is to use a known distance; in this case it’s going to be the distance between the eyes, and use it to compare other distances on the painting. The initial know distance is completely arbitrary and everything else is compared to it. Sometimes I get it so wrong that I need to start over but more often its close enough.