oort, I can create one 4096x4096. 
It depends on your video card capability. But, you are sure when you say that someone "have to add this ability", because that is the limit (4096px).
From the auv_texture.erl
as we can see, Wings gets the size information from the OpenGL (GL_MAX_TEXTURE_SIZE) and uses the minor value between 4096 and that one returned.
[edit]
oort, did you remember about this old thread we have worked: Jpeg resolution crash
Basically, when loading an image for image plane when it was bigger than the value of GL_MAX_TEXTURE_SIZE we need to resize it in order to avoid wings to crash.
It depends on your video card capability. But, you are sure when you say that someone "have to add this ability", because that is the limit (4096px).
From the auv_texture.erl
Code:
draw_options() ->
[MaxTxs0|_] = gl:getIntegerv(?GL_MAX_TEXTURE_SIZE),
MaxTxs = max(min(4096, MaxTxs0), 256),[edit]
oort, did you remember about this old thread we have worked: Jpeg resolution crash
Basically, when loading an image for image plane when it was bigger than the value of GL_MAX_TEXTURE_SIZE we need to resize it in order to avoid wings to crash.