I suggest that you prepare for battle.  Smacking isn't something that I have found is very effective.  It will be a difficult road but if you have the stomach for it you will see a significant change.  Your little girl is trying to be the parent and has successfully been getting her way with her behavior.  When she throws a tantrum it's time for ... Time Out.  If you are in a place where you can ignore her do just that (if not remover her physically to the car or stroller where she is strapped in).  When she is done flailing on the floor or car seat, it's time for time out. "Rule of thumb is no more than one minute per age" but if you've never tried it before you might need longer time outs in the beginning.  Be calm, don't get flustered or yell.  Tell her that her behavior is not appropriate and she needs to sit in time out to think about how she should behave.
Do not talk to her during time out.  Walk away if you’re in the house and if you’re in the car and can't stand it stand outside of it Always make sure your child is safe.  (This doesn't mean lock them in the closet although we are all tempted aren't we  LOL).  If you are in the house and she just gets up, you can either put her back in the same spot or if that doesn't work try a high chair that you can strap her into.  One more step that most people forget - She must apologize to you for her behavior before she gets out of time out.  With a strong willed child this may be a bit trying at first but she must acknowledge her bad behavior and show some respect to you for acting inappropriately.  As she gets more accustomed to this routine, you can use it as a teaching tool to talk about how she could handle it better next time.

This is really phase 2 of time outs.
Using time out with positive/negative reinforcement:
If there are no tantrums, back talk, what ever is the hot issue today, basically no time outs you give positive reinforcement (don't expect this for a while at first  LOL).    A small toy, piece of candy, time with mommy, go to the playground, play a video game, read a book - whatever you know she'll enjoy.  Just something to acknowledge her good behavior.   Now for the negative reinforcement (yuck!)  Please don't use this until you have gotten timeouts working successfully.  It's just too much for them to handle at first. Time outs don't seem to be working or there is a rash of bad behavior.  Some children have a special toy or blanket that makes them feel secure DO NOT use it with negative reinforcement.  Take away something that is meaningful to them, a toy, a privilege, TV time, video game time (never your time).

Be consistent - bad behavior = time out with an apology.  This goes for Daddy too!!  This does not include siblings.  You may discipline her for bad behavior toward her brother/sister but they may not.  YOU are the parents.  Once she realizes that she will not get her way and that there are negative consequences with bad behavior she'll change her tune. It will be very time consuming at first but it will change within a week or two if you keep at it. 

As for the clothes thing, it sounds like she is trying to be a bit more independent, which believe it or not is a good thing.
Picking clothes is a privilege that you can offer to her if she behaves like a big girl - big girls get to pick their own clothes but big girls also don't throw tantrums or talk disrespectfully to mommy.  Don't make it a crazy "we change our clothes 6 times" thing.  Here are 3 outfits to choose from you pick one.  You can be a little flexible if she wants to mix and match, but draw the line about picking from the closet and anything that isn't weather appropriate.  Once she picks that is it, no negotiating or changing her mind.   If she refuses to pick (for whatever reason) use the 1 min rule.  "You have 1 min to pick or mommy will pick."   End of story.

Girls are communicators so talk, talk, talk, talk - that doesn't mean negotiates and there will be times when the answer is no but that is part of being a parent.

Good luck