Hello,
If it is just weaners you are housing then a simple triangle house using the intended 4x8 boards will be well big enough I would say. If your land is new then you don't really need a floor in my opinion, just keep laying straw for them. If it is already muddy then a floor is a good idea, it also helps strengthen a DIY house which you definitely need!
Size of pen really depends on your land. We are heavy clay and have to have enough pens to keep them moving around or they just get sludgey. Friend of mine just up the hill keeps his on a patch of paddock roughly the size of your plan and never really struggles to grow out on just that. Drainage is the key.
Watch your trees aren't anything too special or too small as they may get excavated!
Becca