No sizing issues to worry about :)
I was trying to link to the InTank media basket, but it appears their site is down at this very moment. But they make a great product, and it's already sized to fit in your tank (assuming you get the IM 20L). It fits where the sock would go perfectly. It replaces the sock, and in my opinion gives you much more efficiency and flexibility vs. socks. Keep in mind, I don't like socks, and that's just my opinion on them; many do use them. I don't want to deal with cleaning socks all the time, I use a media basket with Poly-Fil on the top, which I can easily pull and replace every 5-7 days. It couldn't be easier or cheaper, in my opinion.
Here's a link here discussing the InTank vs. the AquaGadget media baskets. One costs more, but I believe they are fuctionally the same; just go with the AquaGadget here.
Looking to change out the filter sock on my IM20 with a media basket. There's a inTank media basket for $45 and IM has one for $23. Any big difference between the two? https://www.innovative-marine.com/shop/CustomCaddy%E2%84%A2-Customizable-Filter-Media-Basket-w-Media-Desktop-p184822924...
www.reef2reef.com
Here's the AquaGadget media basket right through IM. Whatever you get, just get the one that's sized for the "Desktop" series of IM tanks (ie: their 10, 15, 20L and 25 laggon AIO tanks ). These media baskets fit all of those tanks perfectly:
Shop Now | Innovative Marine's Official Store | Free Shipping on all orders* | NUVO aquariums and saltwater reef supplies.
www.innovative-marine.com
Here's the Poly-Fil that I like to use for cheap filter floss. Buy this at sewing/craft stores, or at your local Mega-Lo-Mart:
Buy the The Original Poly-fil® Premium Fiber Fill Bag, 20oz. at Michaels.com. Crafters have made Poly-Fil the best selling polyester fiberfill in America.
www.michaels.com
I would suggest loading the top part of the media basket with floss/Poly-Fil. You can reach this to pull it and replace it often, you don't even need to pull out the media basket. In the middle section I'd run carbon. In the bottom section I'd run some plastic or inert ceramic bio balls. The bio balls are not meant for additional biological filtration. In a marine tank, you want your rock and sand to be the main bio filter. These bio balls are there to be seeded with bacteria in the event that you have to very quickly setup a quarantine or hospital tank. Having pre-cycled bio media to use for this reason is a huge safety net in this hobby. Plastic or ceramic bio balls would not interact with any medication you many necessarily need in a QT/hospital tank.
And that's just the first filter section chamber :) There's baskets to turn the second filter section of these tanks into a refugium to grow beneficial organisms, and/or to potentially reduce nutrients in the main tank.
While I was posting this and looking for links to these media baskets, I noticed there's a lot of sellers on Etsy selling 3D printed media baskets now - check those out! Again, just get the size for the "Desktop" series of IM tanks, and they all will fit.
I would really encourage you to get this IM tank you're thinking about. If you have the room, consider the 25 gallon lagoon tank. It's short, but the front to back depth is awesome. If not, the 20L is a fine tank. There's other choices, but you can't go wrong with an IM AIO. Members here run these tanks from brand new beginners to experts we all look up to. They may a quality product that's well received and well supported in the community.
I hope that helps, sorry for such a long reply :)