When shopping for bar and chains ensure you are comparing similar qualities vs the price. You've got a good saw so don't be afraid of buying decent quality parts.
Laminated bars will be more economical than ones with a replaceable sprocket nose.
My 500 came with an 18" bar in 325 and it worked ok. For limbing and cutting the firewood I have around here I soon realized a 16" bar was more than sufficient and would ease maneuverability. The 18 was replaced with a new 16 and noticeably improved the balance and handling. Over time there was a glitch however...
Irregardless of which bar, chain, type of wood or cutting condition was encountered, each day the oil hole in the bar was ALWAYS, half clogged. Long story short I converted to 3/8 chain (using same bar and similar type chain) and have never looked back.
Sounds like you have a shop locally. My advise is to prepare yourself by searching online (and thru AS sponsors) and learn the prices. Then ask your local dealer to mount a 20" bar and see if you like how it feels. You'll probably find it's more bar than necessary.
You may also want to consider getting a second chain and a new sprocket rim.
Enjoy your new saw.