I'm trying to render an image 15,000 x 8,000 pixels but it seems the highest i can go is 5,000 px on one side.

Is there a setting to allow me to increase the maximum render dims? thx

I didn't see anything in "poser.ini" file, so I make simple scene (a box w/window 375x200) and render to 15000x8000 and it works OK.

do you get some error message?
granted, I only use simple scene for this since render time goes up by square of dimensions

You will probably hit  the limit of your computer's resources (RAM, mostly) before a specific limit by Poser.  I often find that I have to resort to a 64-bit system to render a 2560x1600 wallpaper (the 32-bit systems just can't bring enough RAM to bear).

Scene complexity, high quality render settings, and large pixel dimensions will all increase the burden on your system.

Wipe, your post prompted me to do a test. I ran a 12,000 x 12,000 pixel render in Poser Pro last night - a simple background, 1 V4 character - which ran and completed with no problems; I'm guessing an 8-hour render time.

I've set up a more complex scene at 15K x 15K but I'll let that run over the weekend.

Incidentally, at this resolution, if your figure more or less fills the scene, you start to see all sorts of little glitches (JPEG artifacts, texture stretching, node noise and so on) that wouldn't be apparent on 'normal' renders.

The last time I needed a really big render (6K x 8K) I did it in Vue, and it completed in under 2 hours.

Do you really need to render that large? If you're trying to hit 300ppi for printing, you probably don't. Aim for 200ppi instead - the printed results will be virtually the same.

Imagine your file size after such a render!  I believe postworking will become a substantial problem if you bring it into a 2D program.

why do you need it so high?

12K x 12K = 412Mb. Not a problem for me, I routinely work with image files up to 3Gb in size: Photoshop CS4 running in 64-bit is a big help here.

Image shows complete render with detail at 100%.