Do you want 5 gallons post-boil, 5 gallons into the fermenter, or 5 gallons into bottles/keg? There are volume losses all over the place that you can consider.
1. Boil-off, anywhere from 10-25% depending on kettle, heat source ,etc
2. Cooling loses (4% I think). Boil hot wort takes up 4% more space than 65ish degree wort.
3. Hop absorption and kettle losses. This is the volume lost in hops, hot, and cold break in your kettle. I'm not going to try to estimate this one, it varies depending on the amount and type of hops, amount of cold break, and your kettle configuration.
4. Trub losses each time you rack your beer. Volume lost to trub after primary fermentation. I would bet 1/8 to 1/2 of a gallon is a wide estimate on this.
There are probably more, but I think I hit all of the major losses. If you take good notes on your first couple batches you'll have a good idea on the losses in your process and you can adjust your pre-boil volume accordingly.
With all that being said, based on my process I shoot for about 7 gallons pre-boil volume. Typically this yields 5.5 into the fermenter and a full 5 gallons of finished beer (I almost never secondary). I adjust up or down depending on the length of boil, amount of hops, and gravity of the beer (more beer = more trub loss in my experience).
Hope this helps.