Using the collate textures and altering the slider will resize the textures but it will do it for ALL textures.

Setting it to 2K means that the largest texture size will be 2000 x 2000 pixels, setting it to 1K, the largest texture will be 1000 x 1000. Any texture whose native size is lower than this will not be affected. Reduced textures are proportionally the same as the originals.

Bear in mind, you may not want to do it for all textures, in my case I just reduce the textures for background characters and their clothing. Eyes generally have the same size textures as torsos, which I feel is a bit of an overkill in some cases, so I may reduce them to 500 x 500 pixels as opposed to 3000x 3000. I am currently planning a tutorial on my workflow using Irfanview, as soon as I get the chance.