It’s a pleasure to meet you. Your work is amazing!
About the rendering issue, you might want to try rendering in passes at full res and compositing the channels using different transfer modes in Photoshop. This is what many renderers do internally anyway, but this will give you much more control over the final product and give you a little more artistic flexibility.
If that doesn’t work, or if your renderer has a maximum pixel limit, you can break up your rendering into eight or so sections. Here’s how it works:
First, render out nothing but the alpha channel. This doesn’t have to be at full res but it needs to be as big as you can render it. Next, create a numbered grid of eight or so squares in Photoshop, matching the proportions of your intended rendered image. Each square can be a different color, like red and green, but they should be labeled.
Second, use the grid as a background plate. If your 3D software allows you to use a background image along with an alpha mask, then this would be best and you can ignore the very first step.
Next, drag a section marquee around the first background grid square (with your model if the foreground, of-course). Make sure to go a couple pixels beyond the grid lines. Some if not most of the grid squares might be obscured by your model but the important thing is to see just two intersecting lines so you can line them up in Photoshop. Render this section at full resolution. Do this for the remaining squares.
Finally, in Photoshop, create a file with the same proportions and desired resolution and up-sample just the alpha channel, if you couldn’t render it along with the background, and use this as a layer mask (reveal all) for the foreground image. If you could include the alpha channel along with each grid square, then ignore this last step (this would be best). On a layer, bring in your grid (the one you used for your background image) and size it to fit proportionately. Last thing – bring in each full-res rendered section as a layer and align the over-leaved edges of the grid lines. It might be helpful to turn down the opacity of each layer while checking the alignment.
Hope this helps!