Every awning manufacturer seems to make a slightly different size, and you should allow for a tolerance either way. For example, if your A length/size is 975 (see handbook), aim for something that allows for 950-985 and so on.
If you go to the Isabella website, they have a calculator on there for their own sizes, which would give you a clue.
For a 1000cm, try looking for 975-1025 or somewhere inbetween that.
If you try for an exact size at the lower end, you may find the awning is a little tight when pegged, conversely, at the upper end, it would be too baggy when pegged.