I normally see 1/10th to 1/8th of a gallon per lb. water absorption rate, so at that rate 23 lbs. of grain is going to absorb somewhere between 2 and 3 gallons of beer. I don't know if others have observed different ratios. Additionally, if you want to end up with five gallons post-boil, you'll need to remember that you're going to boil off something between 1-2 gallons during your hour-long boil. Again, this is highly dependent upon your equipment setup, procedures and the ambient conditions (temperature, humidity, wind, etc.) However, I don't think you're going to have to worry about ending up with too much wort. You may want to make a rather stiff mash (some people will mash as stiff as 1 qt/lb or even .75 qt/lb) in order to give yourself enough room to sparge.
The alternative would be to do a no-sparge. If you mash in at 1.5 qt/lb, after grain absorption you should get around 5.9 gal. of wort, which after boiling down should get you into the neighborhood of 5 gallons, depending on your boil-off rate.
Either way, whether you go with a stiff mash and sparge or no-sparge, you should anticipate your efficiency being lower than what it would be with a normal gravity beer--this is common with high-gravity beers. One way to not waste all the sugars left behind in your mash is to go no-sparge for the big beer, drain off your wort and boil, then sparge and make a second, smaller beer with the second runnings from your grain. Check out "partigyle brewing" for more details, and braukaiser.com has a great spreadsheet for helping you calculate your outputs--I've used it several times in the past and I hit the numbers right on almost all the time.
Plugging your numbers into braukaiser's spreadsheet, it looks like you could do a partigyle and make 5 gallons of post-boil wort with an OG of 1.096 from the first runnings, then make a second, smaller beer of 5 gallons (post-boil) with an OG of 1.039. Partigyle is a fun way to make a bunch of beer all at once, it just requires having a big enough mash tun. Hope that's helpful.
