Time and Temperature to bake small fruit cakes
Recipes for fruitcake vary tremendously, without more information there isn't any way to say how long and at what temperature. I looked at several recipes, and none called for a bain marie, but some do recommend a pan of water below the cake to provide moist heat.

You might find this Food Network Recipe helpful, because it gives directions for various cake sizes, but the directions are for this particular recipe, not yours. If I were using that link as a guide, I would use a temp of about 260-265F and start testing the cakes for doneness at about 1.5 hours.

All of the fruitcake recipes I looked at use a skewer or toothpick stuck in the center to test for doneness. The cake is done when the toothpick comes out clean.