NOOB - How do I use higher res wall textures?

You have to scale the texture. The easiest way is using UDMF format. In GZDB, select the line, and there's an option for "Scale" on the Front and Back properties. Just increase the scale to make the texture appear smaller.

Alternatively if you aren't using UDMF, or you don't want to manually scale each texture every time, you can define a scale using the TEXTURES lump.

Download Kurikai's HD texture pack for ZDoom and drag it onto the GZDoom icon.

To see how it was constructed, check it out in Slade3.

Of importance is the hires folder and that the textures retain the name of the texture

they are to replace.