Crutchfield has a user guide, will tell you what fits where. Front doors are 6 1/2's, rears are 6X9's and dash tweeters are 1".
Try cleaning the battery posts really good, then charge up the battery. The "high humming noise" is alternator whine, it will vary in freq or pitch when you press the accelerator. Also check where they grounded the radio, do NOT use the ground in harness.