For tanks, 5lb is pretty standard and a good place to start. I have 5, 15 and 20lb tanks. And a set up for paint ball CO2. For me, the price to fill 5, 10 and 15 is the same. 20lb costs more to fill.
To fill bottles, I use a piece of racking cane and a drilled stopper. I think it's a number two stopper. Jam the cane into a picnic tap, push it into the bottle so the stopper seals, start filling, burp the stopper to release pressure as necessary. If I'm bottling for competition or long term storage, I will flush the bottles with CO2 from a second tank (or a split line) using a pneumatic air gun and a hose.