Filling kegs

I have never kegged a beer before and i am planning on buying a keg to convert my mini fridge to a kegerator. I only brew in 2.5 gallon batches so I was planning on buying a 3 gallon keg - however my fridge is not big enough for the 3 gallon keg. So I figured I'd get a 2.5 gallon keg

My question is: How full do you fill kegs? If I rack 2.5 gallons of beer to a 2.5 gallon keg, will the keg be too full? ie does the keg need a little extra 'breathing room'?

some people fill up to the top, but there is a gas line that sticks down about 2". having the gas come in on your beer is no prob but if you somehow get back pressure you will running beer into your regulator which is not good.. you can get check valves that stop that tho. i'm not possitive on the volume tho.. i would be guessing when i say its prolly 2.5 under the gas line. best way to figure it out is to measure out 2.5 glas of water somehow and see where that puts ya

To carbonate readily the level of the beer should be below the in-tube. That maximizes the contact area between the beer and the CO2. Doesn't matter if you are doing set and wait or force carbonating. For a 5 gallon keg, that's about 5.25 gallons of beer. You could go a bit higher if you are priming the beer, but you run the risk of backflow into the regulator when you connect it.