Shrink the miles and feet
(OK, that really means shrink the people)
The tricky thing about calling a rabbit a 'smeerp' (Reddit) is that sometimes you want to call a rabbit a smeerp, because the characters' ordinary experiences really are the right mixture of similar to and different from our own to warrant it. You want human-like characters in impossibly large trees, so I think this qualifies.
Measurement units originate as analogues to human experiences and are chosen for their usefulness at the time, not because some giant interstellar standards agency says we have to measure everything in Galactic radii.("How far is it to the corner drugstore?" "Oh, go down the street about 0.0000000000000000013GR and it's right there.")
The 'foot' started out as, well, the length of a human foot, and the 'mile' as a thousand paces marched by Roman legions.
If your people have evolved to be what we would call 'very small' in what we would call 'normal gravity', then a tree doesn't have to be physically impossibly large in order to have people measuring its size as 'very large' relative to themselves. Even large enough to build a city in. If your people are the size of, say, termites, and their feet are about 0.1mm long, then that '1.5 mile' diameter limb would be 794mm in our reckoning, which is still sizeable but by no means impossible.
James Blish, the likely originator of the smeerp concept, wrote a famous story called "Surface Tension" that has tiny people having human-analogue experiences like this.