All force fields are built as Force Walls (see HSR 5th p.116-117) with the Limited Shape, -¼ and Self Only, -½ limitations. The shape can be a line, arc, cylinder or sphere, but once defined it cannot be changed — in other words, if you want to be able to create walls in different geometries, you'll need multiple generators.
They always cost END to create and maintain, and will therefore need to be connected to some sort of energy supply, or are built with Continuing Charges to simulate batteries. In any case, they are always created by a field generator, which is defined as a Bulky Fragile Focus.
The size of the generator is directly proportional to the size and strength of the field it generates. The smallest controllable defensive field generator currently available masses about 10kg, and is roughly the size of a 21st century car battery; this will create a 2m x 2m 2PD flat field immediately in front of itself.
NOTE: much smaller generators can be constructed, but the fields they create are not very controllable — they tend to build and collapse very quickly. The technology has its uses (cf. Field Grenades) but so far the miniaturized generators have proven useless for defensive purposes.