The issue is not just the tank size -- it's the food availability. Most Mandarins eat ONLY live foods like pods. Tanks take a year to establish enough of a pod population to feed the mandarin. Some people will put them in after 6 months, and find that within a month of adding it, their pod population is gone, and they're mandarin dies of starvation.
If you insist on one, make sure it eats frozen mysis -- do NOT leave a store with one if you have not seen it eat frozen mysis. I would be wary of an lfs employee telling you that it eats frozen food without seeing it with your own eyes.
The other option is to buy bottles of live pods. But you'd have to really keep buying them.