Bigger is always better. There is a cage size calculator that tells you the recommended number of rats based on the dimensions you input, assuming you use the space well with lots of hammocks and levels and such. I'm sure you can find it on google pretty easily. A single critter nation is great for 5 rats. It's the main cage I use for my 5. A single critter nation says it's good for up to 6 rats too. You can usually find critter nations for sale online or look on websites like craigslist for a used one (we got our double critter nation used off craigslist and it's been pretty great!).
Free-feeding is usually best for rats unless they have special diet requirements. Just a full bowl or two of a good quality lab block like Oxbow Regal Rat would be good. Don't forget to include healthy fresh fruits and veggies sometimes!
I think 5 is maybe my favorite number of rats to currently own! It's great seeing how different relationships develop or change within the group, and see who's best friends, and who's just buddies, and then see those relationships change and see how they bond more with another sibling and how they interact differently with other rats.
Just make sure you're able to definitely afford that many since having to use antibiotics is pretty common with URIs or snuffly wheezyness. And especially since you have 5 girls, you should save money for future surgeries since most girl rats develop tumors when they get older, usually they start getting them around 1.5 years old, but sometimes even younger. And if they're all the same age, they may all start getting them around the same time and all need surgeries to remove tumors. Spaying a good alternative. It greatly reduces the risk of female rats developing hormone driven tumors like the common mammary tumors, or pituitary tumors. Spaying may also save you money in the long run so your girls that get multiple tumors won't have to go in for 2 or 3 surgeries each to keep removing tumors that develop as they get older.
I can't comment on the Martin cage as I've never had one. With my 5, I have to be careful of little tails, toes and faces sticking out of the doorways as I close the doors. I've pinched a toe or two more than once!