The ideal keg size for a 23l batch is 23l or as close as you can get above that.
You can use a larger capacity keg with no problems, but you will end up using more CO2, since you have to pressurize a larger volume.
Using smaller kegs is also possible, but a bit of a pain since you have twice the work to do cleaning, sanitizing and filling, and again, the one half-filled keg represents a waste of of CO2.
Corny kegs are popular with homebrewers, which hold 19 liters. I used to brew UK gallon sizes (23l) but switched to the typical 5 US gallon batch (19L) when I moved over to corny kegs. Since you'll have about 3 liters left over from your usual batch, you could fill two 1.5l PET soda bottles first, with a little priming sugar, and then fill the keg afterwards. This will avoid wasting any beer from the batch or using more CO2 than you need, while also giving you a couple of bottles over, which can be handy to give to a friends, take to parties etc.
Do you have place to keep the kegs chilled, and hooked up to CO2? Kegs are certainly a lot easier than bottling, but they do require the right environment (temperature and CO2 pressure) to provide a good pour.
