Instar,
For a temp fence for peas you can go up to 15 feet without much problems using regular steel woven wire.
For a permeanant stock proof fence I go every ten feet with at least two and half to three feet below ground and a wooden post set about every fifty feet for additional strength. I have seen a lot of people try using only steel T posts, but to get the strength they end up setting them every six feet!
Instead of putting your fencing for the chicken yard onto steel posts use wooden ones. You can put up wooden board fence, which will look nice around the garden, and then you can staple the chicken wire onto the boards. (I am talking about the small wire fencing staples, not the staples you use in either a staple gun or tack hammer.)
A lot of the spacing and depth of the post depends upon the type of soil, the wind/snow/ice load your area gets and the kind of fence you will be installing. Good luck with your project!