Actually just read again - its by your house so in the curtalidge and your asking about permitted development. - doh
its far too big for permitted development.
A "rough bit of land at the side of the house to be used for horses and sheep" sounds far to big to be included as curtilage.
However, in the unlikely event that it
is part of the curtilage, then a 40' x 40' shed would not be too big (provided it took up no more than half of the curtilage). It would certainly be considered as permitted development and as such would not need planning. There are, however, restrictions as to siting and height of the building.
Do a bit of research on the internet and find out for yourself the definition of curtilage and its permitted rights. But whatever you do - do not consult the council planners. You might be lucky and get a rare helpful one, but most are not worth talking to and are very negative. For a successful outcome you are better reading up or talking to a professional planner. Mine charges £65/hour and is worth every penny. Do it once - do it right.