I guess it would be difficult to have a very relaxed riding position without a smaller size than the large ?
Am I missing something here? A "very relaxed riding position" for me means near or at least much nearer upright (itself a rather loose word as for the protection of your spine you need to be a few degrees off perfectly perpendicular to the ground).
A near-upright position (with flat bars) then implies a shorter top tube than for drop bars/a flat back, plus possibly a different head-/seat-tube angle.
A near-upright, very relaxed position is possible on a longer top tube with a short stem and swept-back handlebars, the type called North Road being particularly ergonomically sound, especially if the seat tube and head tube angles are slack, say 67-68% to give your legs some leverage with your body nearer a straight line with the seat tube over the bottom bracket. Since the grips on the North Road type of bars point downwards rather than being parallel to the road, the length of the steerer tube also enters into the adjustment of the same frame for the rider's comfort.
On Thorn bikes of the immediately past era, if the rider bought the frame suggested for his height and preferred riding posture and chosen handlebars, most of the work was done for him by Thorn's plethora of thoughtful size/attitude/component combinations -- making the customer responsible only for sending in very specific and accurate body measurements. Basically, Andy Blance had eliminated a huge part of the mistakes made by would-cyclists whose bike end up hanging in the garage gathering dust.
My own opinion is
not that all this makes the bike on Ebay under discussion a dicey buy, but that, if it is approximately right, the new owner can adjust it to his preferred riding position with a thoughtful choice of components, though under the restraint already mentioned that the cost of new components doesn't make it as expensive as ordering a new Thorn bike that fits perfectly out of the box. As examples, I have bikes of 55cm, 57cm, and 59cm, all bought as the nearest smaller available size to what would be ideal (it's a mistake to let a salesman talk you in a larger bike than you need) and ranging from rather quite small to 2m long overall, huge, all adjusted by component choices to the same rider (me) fitting them in exactly the same position to within an unnoticeable or at least easily assimilated 1mm. The key in each instance was an uncut, long steering tube or the availability of a steering tube extender of decent engineering probity, and quite a bit of patient tech sheet comparison, aided by old fashioned fit and try. An indispensable aid to such methods is a relatively long swiveling stem, which in conjunction with a long or lengthened steerer tube adjusts both the height and the reach to the handlebars before you take out your saw or angle grinder to cut the steerer tube; several of my bikes actually still have the adjustable stem fitted, because I haven't found a stem of exactly the correct fractional value in length and angle, or because the adjustable stem turns a foursquare commuter/touring bike into a record-setting racer at the flip of a lever. (In my younger days, I set my own ton-up, kph, downhill, truck-assisted, personal record with that stem putting the handlebar grips down beside the fork crown.)
