Look at the finished dimensions for the trim that you like. It should have a dimensions for the rough opening and also the diameter of the trim (outside to outside). Usually there's about 1/2' or so of lip on each side of the trim. Here's a link for a trim I found with 7 3/4" diameter, which leaves plenty of slop when installing the housing.

Here is a link that might be useful: 6

This post was edited by mm11 on Fri, Jan 25, 13 at 14:17

You also are very likely to need 'remodel' fixtures instead of 'new work' fixtures.

Remodel fixtures are designed to be installed through the rough opening, unlike 'new work' fixtures.

so i buy the housing and trim separately, got it. so now i'm comparing the depth of the remodel housings i see online to the can (housing) that i kept from the old one. this old one measures 5 3/4" in depth (and 6 1/2 in diameter)...now what? i guess i should mention that i have these lights on a sloped ceiling. when they were installed (1986) it was new construction. but i didnt think to mention about the slope because this can and the old trim (eyeball) didn't look like the ones i see online for sloped ceilings, like an angle cut... the one that i took out were just a regular can shape. i did learn from my searches that i would need a gimbal lamp so i could direct the light straight down, right? so sorry for being sooo uneducated about this whole thing and thank you so much for your responses.

It is not necessary to use a gimbal fitting to direct the light straight down.

For example
Halo H47ICAT 6-Inch Housing for Sloped Ceilings (not remodel housings)
http://www.munroelectric.com/catalog/halo/h47icat.html

Remodel housings are typically used with horizontal ceilings since the housings need to clip onto the ceiling board in a way that does not put undue pressure on one point and possibly break it.

Assuming that only the guts were removed (unscrewed), the basic can should still be in place. Check the wiring to ensure that it is working fine/ track down the issue first.

There are kits available to convert cans into pendants, for example
http://www.lowes.com/pd_150428-56102-PAN-4100L_0__?productId=3772617&Ntt=pendant+conversion+kit&pl=1&currentURL=%3FNtt%3Dpendant%2Bconversion%2Bkit&facetInfo=

whew! project done. not much of a project after all. i wound up with shallow housing and the satin nickel eyeball. thanks to all for your help