Going by how much is used with a 7", I believe a 5 will use half the DE (25 sq " vs 49 Sq "). The basic factor is that you need to coat the papers with your initial batch you pump thru the filter. I will give my 7" experience, you likely need half. Your syrup batches may be smaller but the papers need coating to work properly.
I start with anything up to about 3 gal hot syrup in my mix tank. I add 1.5 cups DE and mix well. Then I pump it thru the filter and back into the mix tank. When the syrup coming from the filter sparkles, I then stop the pump, move the filter discharge to my bottler or a bulk container (a SS barrel in my case) and I filter syrup into that. If I have more to filter, a second batch (mine are then 6 gal batches) only gets about 1 cup DE and this time it goes directly to the bottler or barrel. Each subsequent batch is the same as long as the filter has not set unused for more that an hour and a half to two hours. The papers get changed either when it becomes hard to get syrup thru or at the end of that session. Each new day starts with new papers. The numbers as I say are for a 7" and the quantities were back when I had a short bank (3 pairs of plates) Your numbers should be at half. Just remember the first syrup thru a new set of papers is a smaller batch with 2x the DE to coat the papers. Then double the batches and use half the DE. Be sure to filter at a temperature at least 10 degrees F higher than you will bottle at. I filter at 205-210 and bottle at 185-187. Filtering at 10 above bottling temp helps remove more sugarsand so when the syrup is further heated to bottle you are less likely to see new sugarsand.
Remember, cut everything in half. Late in the season you will find you may need more DE, the numbers I gave are for early and mid season.
You could also get some info by calling Darryl, he can help with the numbers.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.