Next time use old newspaper pieces like papier mache all over your existing block til you get the size you need! Leave to dry. Sand down if not smooth and then cover with plastic before making piece!
It's a good idea to get some cheap wool felt hoods and then block that first over your existing hat block as if you were making a felt hat. Once done, keep the felt hood on the block, cover in cling wrap and then go ahead and block the hat as if it was a regular hat block.
The good thing with using felt is that you can block one or two felts on your block to increase to the desired size and then treat it as a regular block. Plus once you are done you can remove the felt and it won't damage your block. You can also cut a strip of felt to increase the collar on your brim block to the same size as the crown.
AS Kelly says...double and triple block it.
That said, it can depend a little on the style of the block. Domes and simple crowns take this well but shaped ones a bit less so. As you add layers of felt sharper/deeper contours can be much reduced. You can at times re define them by hand once off the block depending on skills and material used for the hat.
To take a 23 up to 24 .5'' you may need to thicken up the block by around 6mm all round, perhaps two 3mm felts will do. If it ends up a little larger, the sweat band or petersham inside will reduce a little of the size.
(Block maker at Hat Blocks Australia)