You will need approximately a 12" diameter circle to fit snugly in the canister, similar to a normal filter pad. Since the Poret isn't sold in a round shape, you will need to buy the rectangular shape, cut it to fit with a sharp knife (use the smaller diameter filter lattice screen as a template) and use the scraps in the filter as needed.
I use the 2" thick Poret in 10PPI and/or 20PPI in my filters. The number of pads I use varies depending on how much other media I use. Here are how mine are set up
This is how I use the 10 and 20PPI Poret in my Eheims. Remember the water flows into the bottom of the Eheim and then is pulled up through the various media layers to the top of the filter and then pumped back to the tank.
Eheim 2262 #1
Bottom media is 5L of EhfiMech
One 2" thick 10PPI Poret foam, cut to fit
1/2" of Premium Polyfil
One 2" thick 20PPI Poret foam, cut to fit
5L of Ehfi Substrat Pro in a mesh bag
Eheim 2260 #2
Bottom media is 5L of EhfiMech
One 2" thick 20PPI Poret foam, cut to fit
Double layer of Polyfil Traditional Batting
Ehfi Substrat, Seachem Pond Matrix and 1/2" spiky bio balls that came with the filter used, all in a mesh bag
Eheim 2260 #3
Bottom media is EhfiMech, about 3" deep
One 2" thick 20PPI Poret foam, cut to fit
One 1" thick layer of Mountain Mist Smart 100% Polyfoam
1 layer of blue bonded filter pad
5L of Ehfi Substrat Pro in media bag
Scrap pieces of 20PPI Poret foam left over from cutting round pads for the Eheim