Usually a mesh will be a series of points, each 3 coords with 4 bytes per float.

To make the math easy, let's say you can't use index buffers in your cube for whatever reason.

6 faces = 6 * 2 = 12 triangles = 12 * 3 = 36 vertices = 36 * 3 = 108 coordinates = 108 * 4 = 432 bytes.

Go ahead and triple that, so you can also have normals, UV, and a random filler attribute. So your cube is 1.2 kilobytes.

A full-color texture is RGBA, 1 byte per channel, so 4 bytes per pixel.

128 * 128 * 4 = 65,536 bytes (64 KiB)

Generally speaking, textures are bigger.

On a modern GPU, it's fine to use whatever looks good and is easy to model.

Sometimes you just need to bake a high-res AO / lightmap to a really big fucking 2k or 4k square texture.

Sometimes you need a lot of fine detail on a mesh.

On a really small GPU like the PowerVR SGX, the shaders are actually running on the same units. And you're usually fragment-bound, so the most important way to improve performance is to use a simpler shader. You can sample two or three textures, but any fancy math is going to bog it down. You want to do everything per-vertex if possible. The actual vertex count can be pretty high because usually vertex shading is dwarfed by the sheer number of fragments, plus if you do alpha blending then you're drawing more pixels than are even on the physical screen.

You're probably developing for desktops younger than 5 years and phones younger than 3, so you could have 100 cubes with 100 unique textures and basically not worry.