Normally a 15watt amp will be fitted with an 8inch speaker. Loudness can depend on the brand, some will sound very loud, while others you will have to max out for it to be loud. Price does play an important part. Although it's small, most of the 15watts are still loud enough for you to disturb your neighbours to call the coppers.
Loudness is not proportional, more a logarithmic curve. So don't expect a 30watter to be twice as loud as the 15watt one. It will be noticably louder, but just so only.
Ok, for guitar amps the more important thing is the speaker size. Standard sizes are (4", 6.5"), 8", 10", 12". The reason why you will find that you can more easily sound more close to your favourite guitarist on a 12" speaker, is because most of the songs are recorded on a 12" style amp. It does not mean that you cannot get good tone on a 8" or 10" speaker.
Therefore people normally progress into a 12" setup sooner or later. The good thing about transistor amps (most modern amps are transistor) is that they will give good tone even in low volume settings. So you don't need to worry about no being able to crank enough to get that good tone on a tube amp, so as not to disturb your family or neighbours.
Thats' why for the Smarvo brand amp having a 12" 40watts model selling at only $190 is really one of the best deals around.