As far as strength is concerned, the maximum tenon lengths need be no greater than five times their thickness. Taking this as a guide, 6mm tenons fitting into 'blind' mortises would indicate a minimum leg cross-section of about 36mm x 36mm. Left unshaped, this would lead to what most of us might regard as a rather hefty and inelegant design. However, I started with stuff somewhat smaller than the optimum cross-section.
Using rails of only 15mm thickness allowed scope for refining the design by bevelling the outer faces of the legs and then by tapering them down to a smallish section at their lower ends.
Joint Design.
Plain tenons on the wide rails would imply long mortises that could weaken the structure; hence as is usual, double tenons are suggested.
I have not shown the conventional tapered haunch at the top (illustrated in the inset) because this produces short grain perilously near to the leg tops. The alternative would be a rectangular haunch (also inset) that finishes below the top of the rails. Since its benefit would be limited, one has not been shown on the main drawing.
Even though I can't see it adding much structural benefit, but not wishing to break with convention without good reason, I have shown the usual haunch between the tenons
Avoid twisted frames checking the face sides and face edges with winding rods.
Retain your face sides and face edges (datum surfaces) by marking out the joints before attempting any shaping.
Chop the mortises while the legs remain rectangular in section.
Slightly curve the top surfaces of the legs (here shown as two flats) and slightly radius the top arrises and the outer corners of the legs. Keep fairly sharp arrises on the inner edges.
Gently curve the upper edges of the rails but maintain sharp arrises. Definitions
Mount the completed frame on a flat, untwisted and level surface. Pack appropriate feet with wedges and check the rails with a spirit level. Use a cabinetmaker's mouse or a suitable block to scribe the legs at the minimum required height from the reference surface.
Fix the fabric by means of screws through slender slats that have been rebated to allow for a hem.
Between insides of rails: 175mm and 490mm.
The rails were made out of 900mm (and 500mm) x 15mm (finished size).
The legs at 530mm long were 33mm x 28mm prior to shaping.
I regret that I cannot now find a UK supplier of woven raffia fabric, though a Web search for 'raffia fabric' will presently reveal a couple of overseas sources.