Poser sets aside memory for textures in blocks of integer powers of 2, such that a 4000x2000 map takes a 4096x4096 block.  A 4100x2000 map would eat an 8192x8192 block, because the 4100 dimension exceeds 4096.

     That said, there's nothing wrong with making non-square maps for such purposes as consistent pixels per unit area.  Two things which you can do to optimize memory usage would be to size your maps so that the longer side is equal/less than a 2^integer value.  For example, for an 8:1 aspect ratio, you might make a 4000x500 map, or a 4096x512 map.

     You might also stack three or four such strip maps onto a single image, then use the image map node's scale & offset values to selectively apply the desired strip for each material.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5