having to backwash new sand filter multiple times a week

Just installed a new sand filter after old cartridge filter died (it was 26 years old).

While I love the ease of backwashing the sand filter, I am finding that I have less flow through the system with this new filter even after it is backwashed.

Once backwashed its "zero" is at 20 on the pressure guage, and when it goes up even to 25 the flow just stops. In the 6 days I have had the filter, the flow has come absolutely to a stop on at least three occasions (and does so each time after running the Polaris for any extended period of time).

Is this an issue with the size of the filter?

Could be. Post specs about your pump and filter and let one of equipment gurus see what they think. 20 may be normal for your filter's clean pressure and if so, someone please correct me. But, that seems high for a clean filter pressure. My clean filter pressure is more like 10 or 12. With a sand filter, a pressure increase of 5-10 is indicative of the need to backwash. So, the fact that a 5psi has a diminished flow isn't farfetched.

You say that it happens after you run the Polaris for any extended period of time. You should see a pressure increase when you vacuum, especially if you are vacuuming up much debris. That would be a sign that the filter is doing its job --- removing debris from the water.

20 seems a little high, but I don't think ridiculous...
Mine runs as low as 6 or 7, but I have a 70gpm sand filter (200 or 250 lb, I think) and my pump is rated at 75 gpm I think, so not much extra...
The 20 is the cumulative total of backpressure, mostly from the filter but also from the return lines; did you have to change any piping? and did you use the same size or bigger?

Whoever sold you a 200# sand filter as a 70 GPM filter was deceiving you, either accidentally (because they were ignorant) or deliberately.

To get good results at 70GPM, you need at least a 30" filter -- 600# of sand. Even a 24" filter needs ~350# of sand.

To get definite answers, you need to post pump & filter maker and models, and send photos to poolforum AT gmail DOT com

My Hayward 22" (250#) runs at 16 PSI with the skimmer and 14 PSI using the Navigator when clean. The Navigator stops working around 22 PSI. I always find that the Navigator causes the filter to get dirty much faster than the skimmer even when the pool looks clean and I run it under the solar cover most of the time. Did you initially run the filter in the backwash position? Hayward recommends you initially backwash for the first two minutes to flush any impurities and fine sand particles. Also are you rinsing after the backwash? This prevents flushed dirt from going back into the filter and pool.