Holly good morning and thank you for writing.
With the screen two things get old: the emulsion and the fabric.
The emulsion gets old after about two thousand passes.
The fabric gets old after about twenty thousand passes.
When the emulsion gets old, you can remove it with a specific product called "stripper", wash very well, let it dry, and then recoat it with a new layer of emulsion.
When the fabric gets old, you must remove it, then stretch the frame with a new fabric.
When the frame gets old I suggest you throw it away.

The maximum size of a screen could be  3 m wide by 6 m long.
However the problem is not to make the screen and engrave it.
The real problem is to use it by hand and without a special machine.
For example, it would be very difficult to pull a 2 or 3 meter long squeegee by hand.
I would suggest that it would be better to print your 5 meter sheet in smaller sections on a smaller screen.

It is not important whether you have 5000w or less, what is important is that it must be a UV light.
For example, a fluorescent tube of 36 w is enough to engrave your screen (with whatever emulsion you decide to use) if it is a UV tube.
Screenprintingstore has in stock kits of fluorescent tubes UV lights.
An alternative is to use a special kind of emulsion, e.g.  the QLT Emulsion, which we have in stock. With this you can engrave your screen under a standard cold light (blue light 5000-6000 °K).
I hope this answers your question

Does the problem arises printing big and regular areas? It could depends on the permeability of the paper that soaks up the ink, so the ink itself goes into the fibre without remain on the paper surface. A frame of 120 thread it`s too much for big areas, maybe it`s better a thicker 77 thread frame that allows to a higher quantity of ink to pass and deposit on the paper. You should also try to change paper and silk screenprint using a less absorbent and more compact one so the ink will dry before go into the fibre.