Depending on the gears you currently have would depend on what size tire you want to run, I forget what gear ratio's they came factory with for the 1999's, on the newer trucks though most are able to run 35's with 3.55 gears without much loss in power, no if you are running 37's the tallest gear I would go with would be a 3.92 and that is only if you wanted to, for best performance you would want at least 4.10's and preferably 4.56's. With that said a gear swap on the 99's are not bad at all and can be done by most people in the driveway if you have done it before or can follow a chiltons or other installation guide.
Now I had a 06 and 2012 (I know its not a 99') ram with a 6" and 8" lift and 35's with factory gearing and it was ok, not a big loss in power and actually better mileage on the highway than before but worse in town, still averaged to be about 1 full mpg better on average, this is of coarse specific to that truck I had at the time and may differ for you, however I know others that have experienced similar gains with all kinds of different rams if they do mainly highway driving.
As stated above I would go with the 5" over 3" simply because the cost difference is minimal, and you won't find yourself wishing you had gone with the 5" to start with.
With that said if you need pricing on anything reach out to us we can hook you up with CAI's, lifts, levels, tuners, wheels, tires, whatever you need and the forum members here on the boards get the biggest discounts we have available often times making us the lowest on the net by a good amount
If you have any other questions feel free to ask we are more than happy to assist where we can.