Is your firewood cut into 25" long pieces? I would think your existing bar and chain would be sufficient. Ripping is a much harder operation than "noodling" as TheJollyLogger suggests. You end up with the same product, but the bar is oriented parallel with the stem of the tree. Makes much larger chips (noodles).
If you really want a longer bar and chain, I would suggest you look at what your existing bar and chain are, and then get the same chain pitch and gauge, with extra links.
What is your current bar and chain? Looking at Stihl's site, it looks like you probably have a 16" bar with a 3/8 pitch, 0.050 gauge (1.3mm) 60 driver chain. IF that is accurate, then a 25" bar with a 3/8 0.050 84 driver chain will bolt on just fine. If you end up getting in a situation where you have the entire bar in a log, your saw will struggle to keep the chain moving, chips may not clear as well, and you may not get oil to the entire bar and chain. Back off, let the saw cool some, let your chain spin for a few seconds to help sling oil. Make sure you keep the groove cleaned out, especially the oiling hole. I would also suggest running a full skip chain to help with chip clearing and reduce the number of cutters in the wood. That chain would be a 33RSF-84 Full skip will possibly create a rougher surface in your blanks, but you will just turn that out anyway. It might make getting your baseplate attached to the blank a little tougher but not impossible.