Well first of all why would anyone nowadays use targa to store images? It's either uncompressed or only run length encoded and depending on the image content that might not help at all. PNG is a lossless image format with much better compression.
However if you want to use TGA, just use it because it doesn't affect the build size at all. Unity stores the images in an appropriate hardware supported texture format for the given build platform. If the texture is compressed at all and which max resolution it should have can be changed at the import settings of each texture.