If the load is 10KW at 208V, that's around 48 amps.
I know there are all sorts of fancy formulas for figuring the exact xfmr size, use them if you want. In reality, the low volt side of a buck/boost xfmr will see the current of the load, or very close to it.
Since you're going from 240 to 208, you'll need an xfmr with a 32V secondary, and a 240V primary. It'll likely be 120/240, or 240/480. It doesn't matter which. The secondary will likely be 16/32V. Connect it for 240 in and 32 out. To connect it as a buck, you can look on a xfmrs website.
1.5KVA at 32V is about 46 amps. Too small. 2KVA is about 62 amps. Good if nothing else is to be added later. I would intentionally fill up the new 208 panel, or someone will certainly add load to it, possibly overloading the xfmr. Code sized overcurrent protection will NOT keep a xfmr from burning up. I would also completely remove the neutral bus, as one leg will be 120V and the other 88V.
I really don't like buck/boosting an entire panel, but in this case it might be the best way to go.
Rob.