The impression I always had was that either all of the nukes had already been used, or that the game designers never wanted the PCs to get their hands on a nuke (or to have a nuke used against them), or a combination of the two. However, if you really want to drop a nuke into your game, there are plenty of sites out there with info on this kind of thing. Even though you've probably already got some of this info, I'll post links for those who don't:
Michael Wong's Nuclear Weapon Effects Calculator
Nuclear War Unthinkable?
You Will Survive Doomsday
I had one other site bookmarked, but it has apparently gone offline. Here's the link just in case it comes back up sometime soon:
Carey Sublette's Nuclear Weapons FAQ
Effects of Nuclear Weapons and Nuclear War
With all of this, you should be able to create some rough numbers for the damage and rads characters might suffer at different ranges and whatnot.
As for deployment, I don't know about the USSR, but I know the US had several different versions of 155mm, 175mm and 203mm nuclear artillery warheads available. Here's what I could find on the 155mm versions:
M454 155mm Nuclear Projectile
Length: 2'10"
Caliber: 155mm
Weight: 119.5 lbs
Warhead: W-48 nuclear warhead, 1-2kt yield, 8.75 mile range
XM785 155mm Nuclear Projectile
Length: 2'10.3"
Caliber: 155mm
Weight: 96 lbs
Warhead: W-82-1 fission warhead, 1-2kt yield, 18.5 mile range using rocket assistance
I'm sure the USSR had their own versions of nuclear artillery; some hunting on the net might track some info down on them.
Deployment is trickier. I don't think the US currently has any nuclear artillery anymore (I think we dismantled it all) but in the original Twilight:2000 history, I seriously doubt that would have happened. The game history even hints at the use of battlefield nukes (both ADMs and artillery nukes). Thus, I'd say it's very possible for the crew of a M109 SP to still have a nuke left, or maybe the players find one hidden in the hulk of a destroyed M109, for example.
And then there's always the possibility of finding an unused ADM (Atomic Demolition Munition, a man-portable nuke, the kind of thing the 5ID left behind when they left Czestochowa). Again, I don't know what deployment was like for these, but I'm sure there are VERY few left by 2000, just as with nuclear artillery shells.
Hope that helps.