Please help with horse breeds?

I'm moving into a house which includes the permanent use of the field out back, it isn't exactly a large field maybe 125x125 feet, would that be big enough to keep a horse in to graze all year long obviously i'l be giving it hay too, and in the corner of the field is an old barn which could easily be converted into a stable for one horse, so what breed would be suit for this? also in a field of this size would the horse manure continue to stack up because its small or would it decompose evenly over time, baring in mind that this will be only be a one horse filed thanks