I am looking for a recommendation for what size ceiling joists to put in for recessed lighting. I am restoring a historic home and need to put deeper joist on 2 of the sloped ceilings for insulation and recessed lighting. I was wondering if I should use 2x8 or 2x10 to be safe so there is enough depth for the IC housing.

What kind of insulation is there going to be in the ceiling? What kind of ventalition? What kind of vapor barrier.

IN general putting a recessed light in ceiling with minimal insulation and ventaltion is receipe for condensation and other water problems.

And can lights comes in all kinds of sizes and shapes. Some as shallow at 5.5".

kal152 already stated IC rated.

Next depends on if the fitures themselves are AT (air tight) or if they require a manufacturer's kit to be AT, and where you are placing these (for example in the ceiling of the highest living space between ceiling and attic or eave space. Next, the temperature, ambient; of the air space containing the fixture and wiring.

Each specific fixture will contain information (usually a combination of labeling and insert flyer or printed on the box) of specific clearances required for spacing. You don't want moisture issues up in a sealed and insulated ceiling cavity either so consider IC rated, AND Air Tight marked and/or the manufacturers AT kit for that specific fixture.

Now, the IC rated fixtures are going to have a temperature sensitive shut-off - but you don't want to be triggering this regularly having annoying flickering lights and having them cut-off - so its usually best to provide some heat-sink room, etc.

You also don't mention the type and depth of insulation you wish to use. Also, if these lights are on a sloped ceiling (like a cathedral ceiling, over a stair case, etc.) they tend to require more height clearance for part of the fixture housing/bracket assembly due to their angle of installation.

This is one of those cases where it is best to do some early shopping and pick out the fixtures (old work versus new work) you wish to use and check the minimum design specifications, then ADD more clearance as needed to accomodate your own unique considerations so they don't build up too much heat and have a way for heat to dissapate. Most manufacturers will let you download or send you the specifications for the specific fixture in advance, or ask them at the store to let you open a box and review the sheet and/or copy it for you, then seal the box back up with the original).

If you can afford the ceiling height reduction, I'd plan on going ahead and beefing up with at least 2x10s since the modern 2x8"s aren't nearly as strong as the old growth dimmensional lumber - and these days x10"s are minimum code - so most such fixtures will minimally work in a cavity 9-1/2" deep.

I'd avoid the minis that use high heat producing halogens, and the like, as they tend to heat up very hot and blistering issues with paint surfaces and such can be a hassle - also finding a zone for a transformer, etc.