I was wondering when you create a typeface do you stick to specific widths and proportions for certain letters. If so how do you got about this other than all b/d/p/q etc widths being the same how would you go about this.
No doubt you are aware of monospace fonts, where each glyph is equal in width. Contrast that to a font like Futura, where there are extreme variances in width between an 'm' and an 'i'. So, like @DA01 says, it's up to you.
If you're not REALLY good with typography, then definitely reference other fonts while making your own. Unless you're doing something really weird, your font will have similar characteristics to others, and you can see how they do it. I created glyphs for a really old-fashioned-looking font once, and while it looked different than fonts like Times or Chaparral, I spent a lot of time looking at those. When you zoom in and look at glyphs, you'll find some interesting quirks that are often worth imitating. For example, even though you'd think that 'p', 'q', 'b', and 'd' are just flipped and mirrored glyphs, they rarely are. An 'o' is rarely a perfect circle. And so on.
If this feels like copying, it's okay. Typography has evolved through the years and many fonts are inspired by others. We humans have somewhat of an unconscious expectation of what our languages' letters are supposed to look like, so you don't want to deviate too far from that.
There is no rule for this. While type design is a craft, it's also very much an art, and decisions like this will ultimately come down to aesthetic calls--and often subjective aesthetic calls.
In addition, typefaces come in all sorts of widths and shapes and flavors. The bigger issues is overall visual consistency, rather than technical exact-width measurements.