The following charts give a rough idea of the height that the average British tree should be in scale.
1ft 7 in
2ft 3 in
2ft 10.5 in
Oh Yes, and for those that are really interested in TY scale or 1/900 the average 65 foot tree should be 0.8667 inches tall!
So as you can see, when modelling, the vast majority of people undersize their trees.
Size is not the only important facet, the structure has also to be suitable for the scale.
For example; you could have a tree six inches tall that is a sapling in G (24th), but would be a full height tree in N (144th). Obviously there would have to be a difference in their structure in order to be realistic for each individual scale.