Hi Darryl: My most recent garden shed is a 10' X 16', from which I leaned a few things to pass on.
2X6 walls 16" OC are a bit much, IMO. 3 of my 4 walls have shelves/brackets for lumber, tools, hardware, etc. I did it by supporting the front of my shelves with 2X4's or 2X2's that go to the 3/4" thick floor. 2X4" walls should be fine while following Paul's advice on rafters. I would also definitely install skylites or windows (if you have the wall space) for much needed light. Also, make sure to run a dedicated 20 amp circuit to the shop and allow plenty of headroom to move those long pieces of lumber around in. One other thing, make your door nice and wide. I made a 40" door mounted with 3, 4" hinges. Works great!