You look for the same characteristics in all stock, which is usually easier to find in wild growing trees and shrubs that have been browsed by animals or beat up by the elements. This works for urban stock as well, except we are the animals who constantly trimmed the plants.
Look for good trunks, nebari, branches, foliage. As with all plants, faults can be used or corrected over time, but having the thick trunks and such that only comes with many years of growing in the open ground is a excellent reason to collect, these attributes are very hard to find in nursery stock.
How big is too big? Good question of which I am sure that everyone will have differing opinions on. As you can see with the cedar above, it was a five foot high shrub when I collected it, it is substantially shorter now and will be worked down over the years even more. The biggest enlightenment most bonsaist go through at first is when they realize just how big all those bonsai they looked at in all the books really are.
Good luck,
Will