Beginner question about how to faithfully quantify an awkward data measurement

Hi all

I do hope I am posting this in the right place. I have some tree species data where tree sizes have been measured but rather than giving the precise measurement, trees have been placed into 5cm size classes. So, class 1 = 0 - 5cm, class 2 = 6 -10 cm and so on.

I am trying to gain a measure of quantites of different species which would be the most faithful to reality without the precise quantities. I have thought of two possible approaches -

1 -Using the mean measurement of each class size
2 - Using the sum of the class sizes recorded for each species

However, both of these methods have the potential to be considerably wide of the mark - up to 50% in certain circumstances. I feel there must be a clever way to do this, but this is getting into maths that I am not proficient in. Can anyone advise?