This is SUCH a good question!!I read somewhere..." Be right...

This is SUCH a good question!!
I read somewhere..." Be right bigger than you are wrong and be right half the time and you will be ok...."
That's pretty much my trading motto LOL
My size depends on my confidence in my analysis.
If price reacts as I anticipated I'll take a small position, I'll add 2 more larger positions as my confidence grows, and the trade is going to plan.

If you want to get technical there's a formula for figuring out the R unit
how much do you want to risk per trade? ( R unit )
If you only want to risk $100 there's a formula for it..
share size = risk amount ( $100 ) divided by the entry price minus the stop price.

Sammi Abusaad explains it from the 36th minute in this video.