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Which size G2 will be the closest match to the Allez?
Allez 56m G2 G2 Large
Seat Tube 530 54 57
Top Tube 564 547 566
Reach 385 384 390
Stack 596 570 595
Head Tube 180 147 173
Head Angle 73.5 71 71
Seat Angle 73.25 74 73.5
Wheelbase 996 1037 1052.7
that didn't format as expected, three columns there. First is Allez 56cm. second is G2 medium, last is G2 Large
The large. The difference is only really in the angles: the seat angle is pushing things forward about 5mm (you can slide the seat back accordingly) and the head angle will be pushing the front wheel forward, slackening off the steering and making for a slightly rearward weight distribution.
The medium will leave you adding quite a few headset spacers and sliding the seat back further.
Assuming you want to retain the same riding position, of course.
Is this the kinesis G2? If so then it looks similarly sized / derived from the discontinued Pro6, one of which I used to have. They are quite tall and non-compact frames so for the correct reach and HT length that I wanted I rode a 57 which left less balls to TT clearance than ideal off road. The G2 looks like it has more slope to the TT but not a load.
FWIW I'm 5'10"ish 33" leg and I would choose the large 57 from that list/
Going on reach and stack the large is closer. It looks like you'll have less standover clearance than the Allez. They also look like quite different handling bikes, the G2 wheel base is quite a bit longer and head angle slacker.
I built a website/database to compare geometry, so I couldn't help but comment. Here's your search on the site...
https://geometrygeeks.bike/compare/kinesis-g2-2018-2019-medium,kinesis-g2-2018-2019-large,specialized-allez-2018-56-0/
Looks like the large G2 is closest. Very similar really, other than the head angle which will push the front wheel out and lengthen wheelbase. The 'fit' will be very close (certainly within sensible saddle positioning and stem choice) but the slacker head angle will make it a bit more relaxed handling... a good thing, I think, unless you like the crit-style raciness of the Allez.