I have a question on how to determine the challenge rating of a monster if it is shrunk.  The normal monster is huge or bigger, how much would the cr go down if that creature was shrunk to medium size.  It would do less to damage with its physical attacks and lose some of them, but still retain its stats.  Just trying to figure out how big of an impact that would make on a monster's lethality.

This is the table for determining how damage is altered by size in 3.5. Just pick a damage level that matches its power at Huge size and adjust to Medium:

http://www.d20srd.org/srd/equipment/weapons.htm
This message was last edited by the user at 05:47, Sun 26 Nov 2017.

Yeah, I know how to alter it.  Just trying to figure out how that alteration would be reflected in its challenge rating.

Really depends on the monster. If it has spellcasting and usually avoids melee, keeping the same abilities and being harder to hit (and having an easier time hiding) is wholly beneficial. If it's meant to be involved in melee, it'll be a lot less effective...

Taking humanoids as an example, you have Medium humanoids ranging from CR 1/2 (Orc) to 3 (Bugbear), Large going from 4 (Ogre) to 10 (Fire Giant), and Huge going from 11 (Cloud Giant) to 13 (Storm Giant). So the reduction in size (and special powers) could be in the 12 CR range (Storm Giant to Orc). If you look at the transition to Ogre from Bugbear, it's a +1 CR, so going from Huge to Medium would be more like -2 CR.

A range of monsters that increase in size as they gain hit dice seem to change size when their CR goes up by ~2 or so, suggesting that a size increase (with extra hit dice for even bigger numbers) is worth around 2-3 CR. This work with the Orc -> Ogre transition. So going down in size without losing hit dice would probably reduce CR by 1-2 per step.

Again, it really depends on what you're looking at and how affected it is by the size change.

You shouldn't really worry about CR. The CR system is nothing more than a benchmark anyway, to get a good ballpark estimate for building encounters. As it is merely a ballpark estimate that needs to be adapted to your individual group per encounter anyway, it isn't really much help to you at all, except perhaps to communicate your experience with the monster's difficulty to other GM's, and if you plan on doing that, then you really need to test the monster out a lot and compare how easily your group defeats the monster compared to other monsters and give a CR rating similar to monsters that gave your group a similar amount of difficulty.

Thanks for the help.  I know, cr is a pretty annoying system, honestly.  Just trying to develop a few scenarios, and I don't want them to seem grossly unfair.  So I was trying to at least use cr as a rough guideline.  But I think I can manage now.