A Mark I coach is 64;6", so upended gives you a good guide as to the size of the average tree for your layout.
The average British Tree is 65 feet tall!
(According to the Woodland Trust)
Most modellers tend to underscale the trees on their layout, so hopefully this chart will help.
3 1/2 inches
5 1/3 inches
1ft 7 in
2ft 3 in
2ft 10 1/2in
3ft 2 1/5in
Not all scales are given, eg ON30, ON3, HON3, etc as, in the main, they would be modelled on American layouts, and this refers to British trees plus this table would take the entire page!
As this is the 'average', it follows that some could be smaller, but also that some should be larger! It all depends on the situation and surroundings of the individual trees.
Research suggests that if you have 100 trees on your layout:
10 of these should be the full mature height for the tree; 40 of these should be 75% of full mature height for the tree; 35 of these should be 50% of the full mature height for the tree; 10 of these should be 25%/saplings of the full mature height for the tree; the remaining 5 should be split between overheight (around 120%) and tiny/saplings/seedlings (less than 25%) of the full mature height for the tree