The few things I know....You should be able to close your fist even before use. If the gloves are leather they will loosen a bit with use over time.
Don't buy them too tight....the blood still needs to circulate

 

The most important thing about glove fitting is that it does not come off your hand in a crash.

Strap on the glove and pull. Try different angles and methods. It should not come off no matter how hard you try. The glove should rip before it slips off your hand.

The only time a glove slips off should be when you undo the straps.

After finding a glove that won't slip off, the rest is simply what feels comfortable to you.

 

I buying them online, but ill try some other gloves at the store. I am very thin, so straps never get very tight on anything, but I have big hands.