Aida is available in many different counts (sizes). The most popular are 11, 14, 18 and 28. The lower the number the less squares there are per inch therefore the larger the crosses of your stitch work. 14 count means 14 squares per inch, 28 means 28 squares per inch, therefore each cross will be smaller on 28 count than on 14 count.
To resize a pattern to a different sized aida take the pattern size and divide it by aida count.
For example:
100(stitches)/18(count)=5.5 inches
Do not forget to add on extra fabric allowance for mounting. I would suggest adding at least 3 inches per side, which would be an extra 6 inches of aida in each direction.
5.5(inches)+6(inches)=11.5 inches
To convert from inches to centimetres you will need to times your inches by 2.54 (11.5 x 2.54 = 29.2 cm).