UV map layout size/ratio.

Hi everyone. I need a texture for some model for to render ar 4000 x 4000 pixels, so I want to make a big texture at time I “economize” size in my texture. I mean, I need a 4096 x 2048 layout or so. So I can Bassicaly avoid unused areas, that is what will happen if I use an 4096 x 4096 layout.

But I can’t do it. If I try in Export dialog then this happens.

Can you help me?