Changing out speakers is a good way to balance tone and volume, there are some caveats, though.
- The speaker/cabinet should be able to dissipate at least as much power as the rated power output of the amp.
- The speaker/cabinet should have a nominal impedance at least as large as the rated output impedance of the amp.
It might not make sense to focus on the actual size of the speaker. Speaker size does affect efficiency (and tone), but you can find less efficient speakers at larger sizes. Also, it might make more sense to focus on entire speaker cabinets, and not just the drivers themselves (it's not clear from your question whether you're already doing this, so I decided to just mention it).
Here's what you can look for in a speaker cabinet to be able to turn the amp up higher without increasing the volume as much:
- Higher impedance: If your amp has an 8 Ohm output impedance rating, then find a 16 Ohm cabinet, if you can. If your amp has a 4 Ohm output impedance, I would go for an 8 Ohm cabinet - not 16 - just to be safe. Doing this will cut the total power output for a given volume setting about in half.
- Lower sensitivity: Perhaps this is obvious. 3 dB less sensitivity at 1W/1m for a cabinet is a noticeable difference in volume, if you can find it. Even a couple dB is worth it. A 6 dB drop would a huge difference.
- Power handling: Again, make sure if you have a 25 Watt amp that your speaker cabinet can dissipate at least 25 Watts (probably not a problem at that rating). Remember if the cabinet is double the impedance of your amp, then you can actually go a bit lower than the rating. For example, if you have an 8 Ohm 25 Watt amp, you can get away with a 16 Ohm 20 Watt cabinet, because the actual power into 16 Ohms would be more like 12 - 18 Watts.
You might find that a 16 Ohm 12" driver in a close-backed cabinet both sounds great and is somewhat quieter.