To render from a 3D soft you just need to care about the size in pixels and don't bother yourself with dpi.
That way for metric size it work like this : an A4 (portrait)document at 300dpi is exactly 2480x3508 pixels.
To know what is exactly the size in pixel (for print) of your document, create a new document in photoshop with the size needed at 300dpi - validate - and then go to menu image - image size and you will see the size in pixels too !
Sorry for my english, but i hope it will answer to your question
