I am trying to figure out which motor to get. I am getting the 70 to avoid having to run new wires but I’m not sure what length to get. The one I have now is 45”. The maxxum only comes 42 or 52. The boat is a 2010 tracker Pro Team 175txw. 

If you fish calm, shallower water then the 42" will likely do fine for you.  That is the standard length.  The 52" is for taller boats, or for fishing deep, wavy water.  I had both for my Bullet.  It was a pain fishing shallow because I had to keep the head so high for the prop to clear the shallows.

You fish the ponds around the area (Cape Cod, etc.) then a 42 should work just fine.  You go up to Winni, Candlewood, Champlain, Quabin) you still MIGHT be ok with the 42, because if it get's choppy enough to make a 42 ineffective, you are probably pushing it with a 17ft tin out there anyway.  That's how I would look at it.  The other thing to consider is if the 52 would hang too far over the side when collapsed and you'd have to re-drill the TM mount.  I had to do that when I went from 42" Motorguide to 52" Maxxum.