When you buy a new snowboard there are lots of things to consider. As well as choosing the correct snowboard for your ability and budget, you need to make sure that it is both designed for the type of riding you do (i.e freestyle, backcountry etc) and also the correct size for you.
We have already determined that snowboard length is predominantly determined by your weight but another important aspect to determine is how wide you need your snowboard to be.
Pretty much all snowboard manufacturers make what they call a 'wide snowboard'. Typically this is about 1cm (ish) wider than their 'normal' snowboard in the same length (e.g. 162 Regular Snowboard 'A' could be waist width 250mm and a 162 Wide Snowboard 'A' would be about 260mm).
The first thing to note is that what one manufacturer terms a 'wide' the next may consider a mid-wide or even standard width. It's a much better idea to actually read the stats on the board to see how wide it is and then from there you can work our if is is wide enough to accommodate your boots.
Boot shell sizes have reduced dramatically over the last few years and now anything up to a UK10 will pretty much fit on most snowboards. If you are a UK10.5 or above then it is worth checking the sizing a bit more to make sure that you won't get toe/heel drag when you ride. If you have a bigger boot but you are determined to ride a narrower snowboard then you may have to open up your stance a little or adopt a wider stance to get yourself onto the wider parts of the board.
As an example, I ride a UK10 Thirtytwo TMII boot. I ride duck 15 / 15 and 23" wide, I can normally get away without toe & heel drag on most snowboards but I try and stay above 254mm because as a general rule of thumb this tends to be wide enough.
As normal, it is always worth heading down to your local/favorite snowboard shop to get fitted up for boots. It's also worth noting that popping down to your local shop just to try their boots on with the intention of buying elsewhere is not cool and does in fact piss off the staff!