My house runs on well water and the main pipe that leaves the pressure tank is 1-1/4" copper. I am looking at installing a spin down filter and regular water filter for the whole house. My question is if I connect a T fitting which reduces the size of the pipe from 1-1/4" to 3/4" and then after the filters back to the 1-1/4" inch size will it have a negative effect on pressure or flow?
You are (and may not realize if you did not pay attention in geometry class) talking about a nearly 2/3 reduction in pipe size (area/cross section) when you go from 1-1/4" to 3/4".
Area of a circle = Pi r squared. 1.23 .vs. 0.44 square inches for the pipes under consideration here.
That will definitely have "a negative impact on pressure and flow" as compared to keeping it at 1-1/4" throughout.