A few months ago, I ordered new sprockets and chain.
At the time, I assumed that the PO had kept the gearing STOCK (bad on me for not checking) so I ordered the standard 15/48 and 428 chain.
When I went to install it, that's when I noticed that he had a 55-tooth rear sprocket.
The bike doesn't seem to suffer from the increased top end, but I know I'd rather have a bit more low-end grunt, so I'd like to get a different rear sprocket.
Problem is...I don't know how much bigger I could go. I'm fairly confident that I could install a 51 with no problems. I left quite a few extra links in the chain. As it sits now, she's close to the end of the snails...I think around 7 or 8.

What do you think...could I squeeze a 55 tooth in there with that length of chain or is 51 about as far as I can push it?

There's a white paper on lower gearing in the Learning Center. Might be worth a read.

It's been said if you want to rock keep it stock! So the 15/48 is probably generally good gearing for most riding. If you're doing a lot of trail riding and technical stuff, lower gearing will certainly help you. We run 15/58 on our XT225s down from 15/50 (stock). BIG help in the junky terrain.

As for chain length you'll need links to cover 7 more teeth (48->55). It isn't quite that easy so leave yourself an extra 2-3 links. Links can be removed much easier than added. And you don't want more than one master link in your chain. 51 or 55 teeth is a hard call as it matters so much on how the bike feels to you. If you go with the lower gearing (55) first you can increase to 51 and use the same chain less some links. The other way won't work as well. Just saying if you're not sure.

I had another look this evening and the snails are on 11...not 7 so I might have enough for a 51.

As for keeping it "stock"...the only bike I've ever owned that I was satisfied with stock gearing, was the TW200 (14/50).
My DR650 came stock with 15/42...WAY too high for any sort of technical work. I've reduced it to 14/45 and find it to be much easier to handle; especially since I don't do a lot of highway miles...but if I did, it's easy enough to swap out the front.

I did mention lower gearing will be better for technical riding in most cases. Hands down. It all depends on what you're riding. Set the bike up for what works best for you. Nothing wrong with mods that improve enjoyment. Good luck.