A 5 Litre (1.3 US gal) pot is the biggest pan I have at present. I would like to make some all grain batches. How much beer can I brew with such a small pan?
In a 1.3 gal pot you probably shouldn't boil more than 2-3 liters because you want to leave some head space to avoid boilovers, which can be a huge mess. With boil-off, you're going to end up with even less beer than 2-3 liters, unless you top off with more water. So it's probably not worth it to brew in less than a 2-3 gallon pot, so you can safely brew 1 gallon batches.
Brewing is a hobby that costs money - the ingredients cost money and the equipment costs money. It doesn't need to be expensive - you can buy economy pots - aluminum or thin economy stainless - and brew great beer. Brewing less than 1 gallon at a time (~10 12 oz bottles) probably isn't worth the effort. You can also get yourself an inexpensive 1 gallon glass jug for fermenting.
Brooklyn Brewshop is a small company that only sells 1 gallon all grain kits. They have a good videos that shows their basic process, which utilizes a strainer for sparging. You could also do brew-in-a-bag (BIAB) with a small strainer bag, like those used for partial mash brewing.