There are a very large number of map projections to choose from.
Which ones are "proper" really depends on what you mean by "proper". Every single one of them introduces some kind of geometric distortion (such as distance, direction or area), yet each also usually preserves some kind of geometric quality. What quality/quantity do you wish to keep "proper"?
WGS84 is a spatial reference system, as you say, but is not actually a projection. Any time you have geographic coordinates (as you seem to have) and have them plotted out, the default "projection" is usually Plate Caree (equirectangular or equidistant cylindrical -- it has many synonyms) projection because the maths are trivial.
The fact that your output is "too wide" suggests to me that you wish to preserve "shape" and that tends to be a quality of conformal map projections. A common conformal projection for the USA is Lambert conical conformal projection. That will give you good shape, including curved parallels.
Experiment with different projections. (Details on how to do that could be given by folks who know the details of Tilemill, SQLite from QGIS.