20mm seam width is standard for a seam with two lines of stitching (and two lines are all that's needed). Good to see that you're placing the cloth sailmaker-fashion, parallel to the leech.

The sewing will start soon, but one question seems to keep me on my toes: how wide should one make the vertical seams of the panels in order to ensure the strength?

20mm overlap? 40? 50? More?

I'm using 190g/sqm polyester (Swela Outguard).

The cloth width is 170cm and the panel width is about 400cm, so I'll have two seams - or possibly three, if I end up using broadseaming (which seems probable). The third seam would be placed at about the 35% spot, i.e. max round.

The  lower battens of the 35sqm sail on my IF are 35 x 2mm, and they cope well. Your sail is so much smaller and the boat so much lighter, so I think 35 x 1.5mm should be OK.  For batten 2 from top, I guess I would prefer the 35 x 2mm version, and maybe even for the boom.

Arne

Alll right, folks - I have started crafting the sail by cutting panel patterns from thick paper.

I'll go for Arne's sail type with AR=2.15 and sail area of about 27 sqm. The cloth will be Swela Outguard (190 g/sqm) in bright yellow :)

The mast issue is still pending, but I would need advice on the batten sizes. They will be aluminium tubes, but would 35/2 or even 35/1.5mm be enough or not? 

Correct, the stepped poles from Nedal and others are flow-formed, like the conical poles. I guess it must be easier for them to run the machine along at the same diameter than to be making the diameter decrease as it runs along. When I mentioned separate tubes and a conical adaptor, I was thinking of the masts I've seen made this way, when full length tubes haven't been available.

If you want to see how it's done:

https://www.youtube.com/watch?v=ZAnzRRjKPpc