Those textures will easily fit on a 1024-1024 texture sheet
Make your texture sheet in photoshop or gimp and then import them into sketchup and apply the textures from that texture sheet you made
Or do you mean after you have applied your textures and you export your model you end up...
I use SBX_313
Click on file/then click on new project
Name your project
then add the path to where you want to save your BGL File and tick the box below,and click ok
Then click on view and click on show background
Then click on file and go down to add map from background...