Both the SCORE code leak and i24 code leak contains source code of what seems to be 3DS Max plugins under \CoH\Utilities\3dsmax\
From the source code. It seems to set a limit on max accepted texture size at 1024x1024 . It can be changed, but to do so the game's executable must be rebuilt.
\CoH\Game\render\tex.h
#define MAX_TEX_SIZE 1024
\CoH\Game\render\tex.c
if (tfh.width>MAX_TEX_SIZE || tfh.height>MAX_TEX_SIZE)
ErrorFilenamef(filename,"Texture Too big - Max tex size is (%dX%d) this one is (%dX%d)\n", MAX_TEX_SIZE,MAX_TEX_SIZE,tfh.width,tfh.height);
\CoH\GetTex\gettex.c
if (reason & 8) {
printf(" Reason: Bad texture (larger than %dx%d or bad file)\n", MAX_TEX_SIZE, MAX_TEX_SIZE);
}