Hi Martin,
A 100'x100' garden is a good start for the first year, then expand it every year after that until you have the size garden you want.
I have gone to my Township Council for permits in the past and it was not a big deal, especially if you are trying to improve the property.
I am not familiar with the type of poplar trees that you have but here it is hard to keep poplar from growing back in the cleared area by suckering or seeding.
My hunting partners and I are looking at establishing wildlife forage plots at our remote hunt camp, but the biggest job would be keeping the trees/brush from regenerating the cleared land and shading out the seeded crops.
Good Luck,
Mike