Firstly, measure the bust/chest size and decide how much extra you would like for ease of wearing.Alternatively, you can measure an old favourite across the chest just below the armholes. The majority of UK patterns these days do state the actual finished measurements and this can vary greatly.Choose the finished size that is closest to the measurements you have taken. If you do have a pattern that doesn't give you the finished measurement you can work this out by using the tension/gauge given in the pattern.For example, a garment with a pattern tension of 24 stitches in 4"/10cm should have 6 stitches per inch/2.5cm. Read your pattern to find the number of stitches for the CIRCUMFERENCE in your size on the main body of the pattern (not the cast on as you often increase before starting the main body of knitting).Divide the number of stitches by 6 or whatever your number of stitches per inch/centimetre and this will give you the finished measurement of the finished garment. Easy isn't it!? Can it possibly go wrong and you end up will a garment you have put precious time and effort into and it still doesn't fit? YES IT CAN!
The above will only work if your tension is correct. From experience I know that a lot of more experienced knitters totally ignore the most important bit of the pattern - the tension/gauge square.Whilst newbies will read and follow instructions in the pattern to the letter a lot of us (supposedly more experienced knitters) choose to ignore this bit of the pattern completely! If you are using standard yarn that you have used before you may get away with it but is it worth the risk? With some of the natural yarns that are about at the moment it is very important not only to knit one, but to wash it and re-measure as not all the yarns on the market at the moment are pre-shrunk.
See my guide on tension squares if you need further guidance.