You don't need to worry about permitted development. Agricultural planning is only about £80. I just applied for and got a building on our place , it's ten acres, I've gone for 60 x 30 because it's a standard size the shed builders use, easy to get competitive quotes, in fact there's a bloke posts on ebay and breaks down all the costs in his advert so you can work it out.
As for size, measure the area your equipment will take..easy, calculate the volume of the bales you intend to store...don't forget you might do 2 cuts or store some for more than a year.
20ft high sounds quite high...how high does your loader lift ? Are you on rounds ?
All stuff to think about.
Apply big, you can always let the council think they've won by coming down in size, and you don't have to build it all at once or at all. Put up 2 bays now, add others as you can afford it, once planning is in place it lasts forever (so long as you've made a start)