Shall I buy a big or small tree?
The market for big garden trees is large in Chiang Mai. People are keen on spending hundreds of thousands of Baht to get a woodland overnight, but not keen on spending 2000 Baht on a good tropical gardening book with advice that would save you a fortune. Luckily there is an article for free illustrating this problem: http://horttech.ashspublications.org/content/15/1/118.full.pdf
The conclusion of the pdf file above is that a cheaper young tree may catch up in size with a larger more expensive tree. I have exactly that experience from jacaranda; tiny seedlings catch up with 2 meter saplings.
My philosophy is to buy a small seedling, saving money, being assured of a healthy condition and feeling the pride of a parent when it grows. A lot of people can not wait and they consider trees to be garden furniture, not living beings.
A few trees can indeed be transplanted also when large, such as Ficus, Plumeria, Bauhinia and coconut, but not without care. Other species may be very difficult to move when large. The 6 months guarantee offered by the vendor sounds generous, but a tree may die over a period of 3-4 years so the vendor takes no risk and can always blame the buyer for insufficient care. In the pdf article above, 58% of transplanted red oaks (Quercus rubra) died within the first year. A reason for a death rate longer than six months is that a fungal rot takes time.
Important factors for the survival of your tree is: 1) age of the tree (the younger the better), 2) presence of damage or fungus 3) selection of soil and terrain (know the preferences of your tree and try to meet those, dig a generous hole and add the soil preferred) 4) selection of time of the year (avoid the hot dry season March-April and the soaking wet season August-September) 5) treatment (water frequently but avoid standing water, shade it) 6) origin (buying rain forest trees such as mangosteen and durian and planting them in full sun here in the dry Chiang Mai is very risky, and so is planting a gorgeous inland tree by the coast where it may succumb to salt intolerance). A basic requirement is to know the scientific name of the tree so that you can retrieve information about it. Do not use a dictionary but a proper flora. If a vendor gives you a name, compare with pictures to make sure he is right.
The principle for transplanting a tree is that you cut some roots first, wrap the root ball and then cut the remaining roots when the previously cut roots start making new root tips. The tree will then be moved to the nursery and kept on the ground. On the ground the root ball dries out quicker which prevents root rot, which is a crucial problem when planted in the soil again. The transportation itself may cause injuries to the bark allowing for pathogenic fungi. The root damage is also a door to fungi and the drought stress in the nursery, sometimes lasting for months and years, may cause drying out of branches which may become bridgeheads for invading fungi.