It's not the size of the fish that is challenging. Well, sometimes you get lucky and hook a really big one in a really small river and that poses it's own challenges, but generally, the fish are a manageable 50-60cm.
The flies used in these small rivers are also the same as in most other rivers. Maybe I won't throw the largest of the tubeflies, but hook flies in sizes 4-6-8-10 and tubes with wings from 3-4 to 8-10cm. It's a good idea to lightly weight the tubes. This ensures that they sink, and fish, as soon as they hit the water and gives you a little depth as well.
Often these rivers have a lot of weed growth, so sinking lines are rarely necessary, and often a bad idea. And intermediate perhaps, but not sinkers. I use a floating line and weighted flies, and may have an intermedaite or a light sink tip with me as well.
Despite the size of the river, a little extra length on the rod is good. There's a lot of green along the river, some of it as tall as yourself, so a 10' rod is really nice when your fishing. And as the river is very small, a 6-wt is enough, a 5 would be fine too. I upline by at least one, maybe two, when fishing these small rivers. I'm casting very short lines, 6-7 meters mostly, and I'm not trying to mimick the feel of a fully loaded rod ny uplining, far from it. In fact, I prefer that the lines still feels "light", because in these small rivers, presentation matters. But what I am after is a little more weight in the line to help cast the weighted tubes.
I keep my leaders on the short side, rarely useing anything longer the 9'. I need some line of the water to make a meaningful swing of the fly (of the river is even wide enugh to make a "meaningful swing). And a longer leader makes casting weighted flies a pain. When the river is too narrow, all you can do is cast, downstream or upstream, and retrieve the fly (upstream in the river, with a wetfly, is sometimes surprisingly effective for sea trout, and with dries as well).
Regarding the fishing described above, the next couple of months is prime time!
Have a great weekend!