The damage is:
level damage
1-3 d8
4-7 d10
8-11 d12
12-15 2d8
16-20 2d10
Speed is based off of the creatures base speed not size. If the base speed is 30ft then calculate as normal if it is 20 use table 3-11 in the PHB. If you can post the natural speed I can tell you the progression.