The maximum size of the field of play is 100m from goal line to goal line, and 70m side to side (330ft x 231 ft approx). There are rules on how big the in-goal areas behind the posts can be, on top of that. Minimum size will vary according to the level of play, and the governing body. I've seen kids under 10 playing on a fully-marked out pitch, complete with miniature posts, that can't have been more than 100ft long.

Largest length allowed under IRB regs is (as Amali states) 144 metres. Minimum is 106 which works out to 347.77 feet. So there's an allowable range of about 125 feet.

a rugby field is 144 meters long maximum. 144 meter = 472.440 944 88 feet

There's no regulation size in rugby and football. It's not like tennis.