You can brew a kit at 15L if you wanted too, it will be higher ABV (more alcohol), and higher IBU (more bitter) as it will be more concentrated. This may or may not work for you dpending on your tastes.
Instead of buying the pre bittered kits you could do extract brews and do your own bitterign by boiling a small amount of wort and add hops to it. The spreadsheet
here is a great tool for working out how much malt extaract and hops to use to meet a given style of beer.
Its not hard to do, if you can boil a pot and set a timer, you can manage to brew extract beer without kit tins. You will need about 2KG of DME to make a round a 5% ABV beer.
Nice simple recipe
15L
2KG LDME
10g Galaxy 10mins
20g Galaxy 5mins
70g Galaxy Dry hop
US-05 yeast
5.2% ABV in bottle
20IBU
Bring 8L of water to the boil, stir in 850g of the malt extract, add 10g of galaxy hops, after 5 mins add another 20g of galaxy hops, after another 5 mins remove from heat and pour the wort into the fermenter, straining off the hops. Add teh rest of the malt and top up with cold water to 15L. Once its cooled to 20c pitch yeast. After 5 days add dry hops, after another 5 days bottle.