I don't know how many pounds of grain you used.. but, I would expect to have a grain absorption of about 0.15gallons/pound of grain. It varies a little.. but, I think that is a good starting point. You will be able to hone that on your next brew based on this one. Also, it is good to know your boil off rate.
My BO rate is right at 1g/hour. I typically boil for 1.5 hrs.. therefore boil off about 1.5 gallons. For a 10# grain bill, I'd lose another 1.5 gallons. So, in this case I'd be looking at about 3 gallons of water loss. Then you have hop loss trub loss to account for. It seems to add up.
So, you might consider, if making a 5g batch.. that you would be wanting to put 5g into bottles/keg. You would have to start there.. add all the losses based on your knowledge of prior brews and come up with a starting point. If you have still lost too much, you can add water to the boil kettle at the the time you are adding your flame out hops or your chiller. You can even add sterilized water into the fermenter to bring your volume up to, say, 5.2 to 5.5 gallons. I'd do it as you were chilling and take a SG sample after mixing to get your OG. Just some thoughts.
Also, consider building a measuring stick to know how much wort you have in your kettle. You can make a stick for ambient water and one for hot water as there is expansion/shrinkage during heating/cooling.