hey Grt,
welcome to the club
what yr is your telstar? i started of driving an 83 626 which was pretty much identical to a telstar of the same yr
anyway i ran 4 gauge along the passenger side, and rca through the centre too, both were quite easy and unnoticable too.
front speakers are 5x7 but can easierly be fitted with 6" as many on here have done
rear door for the hatch is 6 or 6.5 and the rear shelf on the hardtop hold 6x9s.