Anyone know the dimensions of the things?
The range of UAVs I have seen (on Discovery Channel and similar) range from devices weighing less than three kilos to several hundred kilos, maybe even over a thousand.
At the lowest end of the scale you've got very simple, probably electrically powered aircraft that can be carried in a backpack or suitcase and can be operated by a soldier with no flight experience whatsoever. These are typically used in operational recon situations, flying over the top of buildings to see what's behind them and so forth.
At the highest end of the scale you've got aircraft that easily match the size of a PA-28, with turbocharged engines allowing it to go to serious flight levels and stay there for hours on end. They are typically used to keep a eye on larger areas when satellites, either because of cloud or because of orbit issues, are not effective. There was a time when the US military snapped up all Rotax 914s as soon as they rolled off the production line, for the "Predator" but I think they've moved on now to even bigger stuff - and of course with the "single fuel battlefield" you'd expect them to use a diesel engine anyway.