The type/size of field box will depend on what you plan on bringing with you to the field. I have seen people that have their workshop in a field box.

The USUAL stuff you bring to the field in a field box AND USE are:

Battery
Power panel
Electric starter
Glow plug ignitor
Fuel
Fuel pump (I suggest both an electric and a manual as a backup )
Spare props
Spare glow plugs
Spare rubber bands for the wing (Bring 12 IN ADDITION to what you use to attach the wing)
Regular and phillips screw drivers (I suggest a #1 tip phillips and/or JIS for servo screws)
6" adjustable wrench to remove/replace props
Glow plug wrench
Prop reamer
a 6" piece of MonoKote trim tape (the self adhesive type ) for emergency field repairs

While there are a great many more things you can bring, I have found that these items are what I normally use on a regular basis. If the plane requires more than this, you need to bring it back to the shop.

The other thing I would suggest in a field box is a REMOVEABLE section that contains the battery, power panel, and electric starter. It is easier to carry than the full box.