Coffee, this is a tough one to answer. In some areas, depending on your building codes, this type of addition would not even be legal, so first be sure your contractor is licensed and you have a building permit, which should have been signed by an inspector approving the pour of the slab.
Beyond this, it really depends on how far out of square and level the slab is. If it has high/low areas, but is reasonably level overall (outside edge to outside edge), a floor patch is suitable to skim and fill the low areas. High areas are a real problem and expense, requiring use of a concrete planer to remove excess material. Few contractors own these, or have knowledge of how to operate them, but they are available from many equipment rental companies.
If the slab is out of level overall, there are some specialty concrete additives which will improve the adhesion of capping your slab with more material, however, any cap of less than 3-4 inches in thickness will likely not remain solid in the long run.
Out of square is pretty much out of square, without a good long-term solution. You cannot add an inch or two to a side and expect it to stay put; it will crack, crumble and break away almost invariably. If a small amount needs to be removed to acheive square, this can be done with a diamond blade concrete saw, cutting down as much as 4 or 5 inches deep, but what might remain below that on the thickened edge of the slab should remain and hopefully this is near ground level so that it is not very visible. Best of luck to you!