Trichomes in general are features of xerophytic leaves, which reduce water loss by evaporation by trapping water vapor and increasing humidity (as a result lowering the water potential gradient). BUT I wanted to know whether trichomes also reduce water loss by decreasing "surface area" of the leaf, so that less water loss occurs (considering they occur as finger-like projected structures that occur in large numbers)?
Trichomes can serve the purpose you ask about, but certainly not only that. They also appear on a great many other, some very delicate, plants from many different habitats. Decreasing water loss is only one of a number of different functions they might serve.
But with respect to xerophytic plants, trichomes are still a projection from the leaf surface and so, by definition, increase the surface area of a leaf by a significant amount. However, it's not the surface area per sé that leads to water loss, it's increased evapotranspiration which can be caused by and mitigated by many different factors, one of which is, indeed, trichomes.
Some trichomes, such as those on Tillandsia species, occlude or reflect light. This reduction in incident light is primarily what protects the leaf tissue from damage and helps reduce water loss due to heat. Such trichomes may also function, as you mention in your question, by creating a insulating layer of still air around the leaf, thereby reducing diffusion of water vapor away from the leaf and creating a smaller gradient, though this is usually not their primary function.