Rule of thumb for line length is 1' of 3/16" ID tubing (Bevlex) per 1 psi of CO2 pressure the beer is under. Longer lines typically just makes for a longer pour. So, if you plan on having the keg at 10psi, then 10' lines will do the job.
I would advise going with the longest you'll need for the lines initially. It's far better (IMO/IME) to have longer lines than too short (glass of foam). We're not pouring for money, so it doesn't matter if it takes a few seconds longer to pour a pint.
I would also advise getting a solid way of taking a temperature reading of the beer in the keg. Use that information, and this chart, to figure out what PSI you need to have the keg on. The air temp in the fridge is less important since that will fluctuate more widely than the actual beer temp in the keg.