Having just checked my own speakers, a size of a speaker is measured from outer frame edge to outer frame edge across the center line.
The absolute widest part of my 12" woofer measures 12 inches. The cone area plus surround measures 10-7/8 inches.
Does that help?
Also, if you are replacing a speaker, you need to replace both sides. In you case, you need to replace the midrange on both your left and your right speaker to keep the sound balanced. You need speaker of the same impedance, and new speakers with roughly the same characteristics (frequency response, etc...) as you original speaker. Since it is a Mid-range in likely a three way system, that's not as hard as it sounds.
It's a 2 way setup, my tweaters are fine but I play a fair amount of bass-heavy music. I was thinking of replacing both but thanks for answering that!
What should I look for in drivers in terms of specs? Baring in mind the music I tend to play?
Thanks again for your reply
Select 'See More Drivers' (in red) and you will see all the available drives. Most likely these same brands of drivers are available from sources in the UK, though I don't specifically know any of them.
As an example, here is a sample listing of all the parameters that must be taken into consideration -
Now, you don't have to make it this complex. You can just by a reasonable quality woofer of the right size, a reasonable quality tweeter of the right size and type, then an appropriate crossover network, and you will probably have nice functional speakers, but I can't guarantee that they will be any better than what you already have.
Also getting the book 'Introduction to Loudspeaker Design' will help introduce you to this particular brand of 'alchemy'. It is the standard starter book in the field of speaker design.