Situation So Far
So I have recently decided that I wanted to try building my own Cab, this project will be ongoing and won't be finished until summer/fall. The cab in question can squeeze in a 4x12 setup, but I'm currently open to putting any configuration in it.

I'm going to ignore the budget for now, as I can't afford to go out and buy a bunch of speakers at the moment but feel free to suggest something if you like. (try to keep the suggested speakers under $150 please).

I play mainly metal, but I also branch off into other genres and I want to have the most versatile cab possible to support my interests. I don't currently have a head to pair with the cab right now but I'm looking at a Peavy 6550(or 5550), Blackstar Stage 60H but will continue to look and try other heads out as time goes on.

Thus I have a few questions I hope you can answer:

Does having multiple speaker sizes actually add more frequency range to a cab? If they do what is the best combination of speaker size?

Or does the speaker size not matter so much, and it really comes done to the speaker itself and how it's designed that would give me the largest range of sound?
OK, I'm not a guitarist so you'll have to wait for others to answer questions of taste. I'll try and address any technical questions.

Size itself doesn't necessarily affect tone, for example a mid priced hi fi cab might have a 5" driver that goes deeper than a 15" bass speaker and may even have less top end! However if you are home building you have to buy what is on offer and most guitar speakers are 12" so you are probably going to want a 12" speaker.

Mixing speakers in cabs is quite complex and can be unpredictable in the outcome. You could mix two speakers you love the sound of and end up with something you don't like. Two things are going on.

If you have multiple speakers (even identical ones) in a cab then you create what engineers call comb filter effects. Because the pathway the sound travels from each speaker is different the sound arrives at different times. This means the wave from one speaker may be travelling backwards and the wave from the other forwards and the sound cancels. This changes the tone and also makes the tone dependant upon where you stand. It also means a traditional 4x12 radiates a narrow cone of sound. The best arrangement is 2 speakers one on top of the other which radiate a wide flat fan of sound which will spread across a stage so the band can all hear and will also concentrate the sound on the audience.

The 'sound' of a speaker depends upon the frequency irregularities in it's frequency response. The humps and hollows. If all speakers were flat they would all sound the same. If you mix different speakers in the same cab then the humps and bumps won't be the same for both speakers. They will combine to create a new frequency response and you'll get a new 'sound'. Unless by chance the peaks coincide you will generally find the sound of one speaker will cancel the other and you will end up with a smoother sounding more neutral speaker, ie less character.

Given that you are tight for funds I would suggest you go for a 2x12. If you spend $150 on drivers then you can choose almost any driver if you buy one. You can spend $75 ea on a 2x12 and buy nice drivers or you could spend $37.50 on a 4x12 and buy crap. In any case a 2x12 will technically be better and easier to carry. One option might be to build a 1x12 as a test bed and then buy a second driver later, scrap the box and build your 2x12 having gained experience and skill.

You need to start listening to speakers to find ones you like. The Eminence and Celestion websites have sound clips of their drivers. Check them out but many commercial cabs have their drivers so listen to as many cabs as you can and try to find out what is in the ones you love.
Thanks for clearing that up for me I haven't had an opportunity to play through a 2x12 or 4x12, I'm picking up some stuff this weekend so I will being trying out a few combinations. So when I'm checking out heads I will keep an eye on what speakers I'm playing through.

Thanks again, wasn't expecting such a detailed response!