I would really appreciate some help choosing a size of bike frame. I am stuck between choosing the large or the extra large.

I may go down the route of getting professionally checked on the bike, but there is every chance that since this is a frame company a long way from my house I won't be able to get it properly checked before I try and place an order, only testing on equivalent bikes. I have not decided if I am going for flat bars or drop bars, but more likely drop bars. The big issue is that the 57cm won't be available for 10 weeks but the 58.5 is available now.

A: 186cm (6ft 1inch)
B:
C: 87cm
D:
E: 62cm (armpit to bottom of thumb as per picture)
F:

Any help would be really appreciated. My feeling is that the large is the right size, but from my preliminary reading it looks like the extra large may be closer?

Thank you

Based on you measurements, the 58.5 frame is likely closer. Generally, it's much easier to get a size-too-small frame to fit than a size bigger, but it depends on body proportions, flexibility, etc. (One possible issue: is that chart showing "effective" or "actual" top tube length.) I'm 187 cm tall, with a 91 cm inseam, which makes my torso much shorter than normal, compared to inseam. Your measurements seem to be a lot closer to average, so the 58.5 probably won't get you too stretched out . . . assuming you have decent flexibility, no back issues, etc.