With farms you would need to check whether there are any public paths going across the land, as there is a minimum distance you have to keep from them with paintball markes, or any air weapons I believe.
If you are able to fully secure and marshal a game area so that participants, landowners and the general public (including unexpected passers by) are safe, then the amount of space really depends on numbers and the terrain. Speedball tournaments are played on something like 20,000 square feet, with teams starting in range and sight of each other. On the other end of the scale you have big games like North v South played across 700 acres with hundreds/thousands of players on each side.
I hope you don't feel like we're ganging up on you, but I will reiterate that you should be very careful playing outside an actual paintball venue - firstly as it is so easy for something to go wrong and for someone to get hurt, secondly if the media get hold of it then it would badly affect the reputation of the sport at a time when the UKPSF are trying to grow it! How often in recent times has media attention led to hamfisted, over the top legislation? I really wouldn't want this to be the spark that strangles paintball in the UK.