Can I use this sign for a bank check?
No, this specific sign is used for checking off a list, verifying information, or a literal checkmark. A bank check is signed differently, usually by using your thumbs and index fingers to draw the rectangular shape of a paper check.
Does it matter which hand I use as the 'paper'?
Yes, you should always use your non-dominant hand as the flat 'paper' base. Your dominant hand, which you naturally write with, should act as the 'pen' drawing the checkmark.
How big should the checkmark motion be?
Keep the movement relatively small and contained within the palm of your non-dominant hand. It should be a quick, sharp flick of the wrist, just like making a quick mark on a real piece of paper.