Height and girth do not tell the whole story. The overall size of the tree and the effect it makes on the observer is also due to the shape of the crown and spread of the branches. Some of the largest oak and beech trees have massive canopies springing from trunks of relatively modest girth. The greatest trunks may belong to over-mature trees with reduced foliage.
A rough guide is that trees over 10’ in girth are substantial specimens for smaller trees – holly, rowan, birch, cherry, even Scots Pine.
Alder, willows, poplars may exceed 12’. Over 15’ and the tree may be considered big for most broad-leaved species – beech, horse chestnut, lime, sycamore, oak and sweet chestnut. Long-lived specimens often reach 18’.
Once over 20’ the tree is exceptional; over 25’ and it is a real giant – to be expected from exotic conifers like Wellingtonia but very rare for others.