To do this you will have to physically open up each and every texture package while in UnrealEd (or at least the larger texture packages that you want to reduce), and export them all (usually as .dds). Then import them into your editing program, reduce the size, reexport as the same type of .dds file (dxt5, dxt3, dxt1 etc) then reimport them into UnrealEd with the same name (overwriting the existing file). Then save the texture package.
There are some batch export commands, and you could always try using UDE.exe or the UT Package tool (search the UnrealWiki for these) to export all of the textures at once, but importing them would probably be best done texture by texture. Alternatively you probably also just compile a script package, using #EXEC commands to reimport all of the textures into a package (then just rename the package), but I'm not sure that is much faster than just reimporting them via UEd.
I don't know of any good way to do this for the textures embedded in .u files, but those are usually pretty small (mostly icons) so that should not be a concern.
Of course it'd be a good idea to keep copies of the original files elsewhere. Keep in mind that you do not want to reduce the size of any gui/hud textures as there is no guarantee that the code does not use hardcoded texture sizes/coordinates when drawing them (in fact all weapon icons use the same texture with explicit coordinates to only draw a small section).
If I were you, I'd only do this to the playerskins, weaponskins, and some of the level texture packages.