Here is mine. It is single colour (laserjet so the ink doesn't run if it gets wet). I print them 2-up on an A4 sheet and guillotine them to get two A5 bills per sheet. I have a printer with duplex capability as they are two sided. If you don't have duplex, you could just as easily run them through your printer twice - taking great care to ensure they are the right way round for the second run.
I suggest using A5 as your minimum size as, if they are too small, they may get thrown away with junk mail, leaflets etc.