Veranda timber sizes and centres.

Going to put a veranda over our rear patio to make it more usable throughout the year.
The patio is 8m wide and 3m deep and one outer corner is 70cm raised above ground leave (due to slopped garden across the width)
The roof would be poly-carbonate.
My question is what size timbers should I be looking at for the posts, header and joists and what centres.
I would like it to be able to support a person for maintenance if needed I know the polycarb would factor into this but I don't want to go to thick with this as I don't want to reduce light into the patio doors and windows to much.
Thanks.

100x100mm posts
150x50mm headers
100x50 joists at 500mm centres
All tanalised timber or oak hardwood
Need it to withstand snow loading
Person on PC roofing will crack it, need to use crawl boards
Dont forget flashing, gutters & drainage, and planning implications as needed