Frame Size help on Cannondale Synapse

Are Cannondale bike frames bigger than other manufacturers ?I've brought myself a Cannondale Synapse Carbon Ultrega Disc in a 58 frame, but I'm not too sure if the frame is too big for me as it feels bigger than my Trek Emonda S5 which is a 58 frame.
I'm 6 foot with an inside leg measurement of 33.5 inches and looking at the charts this bike should fit but i'm now having my doubts.
I'm struggling to get comfy on the Cannondale the reason I got this bike is that i'm getting on a bit at 49 and have lower back problems and wanted a more comfortable bike and the Synapse ticked all the right boxes but the reach on this seems to be longer, i've put a shorter stem on but still can't get it right.
I'm wondering what frame size other members are using and their recommendations for Cannondale?
I'm 6 foot with an inside leg measurement of 33.5 inches and looking at the charts this bike should fit but i'm now having my doubts.
I'm struggling to get comfy on the Cannondale the reason I got this bike is that i'm getting on a bit at 49 and have lower back problems and wanted a more comfortable bike and the Synapse ticked all the right boxes but the reach on this seems to be longer, i've put a shorter stem on but still can't get it right.
I'm wondering what frame size other members are using and their recommendations for Cannondale?
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https://www.cube.eu/uk/products/road-ra ... hred-2016/
Now comparing that to the 58cm Cannondale...
http://www.cannondale.com/en/Great%20Br ... 8a4d20906f
The stack is the is fractionally higher on the Synapse, 610 vs 611mm.
But more crucially, the reach is longer too on the Synapse, 388 vs 394mm.
I suspect the 58cm Cannondale would stretch me out just a fraction too much, as I also have a lower back issue, which has been much better in the last ~14 months. Flipping the stem (so it points upwards) would help reduce the reach a fraction, while a 1-2cm shorter stem would help further, at the cost of more twitchy steering without fitting a wider set of bars.
Have you tried anything close to 75cm BB to saddle top, with the saddle all the way forward on the rails, with the stem pointing up? Looking at my Cube with ~72cm, that is still going to put your saddle ~5cm above the height of the bars, whereas my bars are ~2cm lower at maximum with the stem in the downward position.
Handy stem comparison tool http://yojimg.net/bike/web_tools/stem.php (flipping my stem would reduce reach by 8mm and increase stack by 26mm)
2020 Voodoo Marasa
2017 Cube Attain GTC Pro Disc 2016
2016 Voodoo Wazoo
I adjusted the saddle forward on my last ride yesterday but it wasn't comfy my knees are aching this morning.
Suggest you do indeed return to Evans and try a 56.
Kinesis Racelight 4S
Specialized Allez Elite (Frame/Forks for sale)
Specialized Crosstrail Comp Disk (For sale)
You can't compare bikes by simply using one number - you need to look at the geometry or at least look at stack and reach figures.
The Edmondo figures for a 58cm frame are: stack 59.6. reach 39.1, angle 73
The Synapse figures for a 58cm frame are: stack 61.1. reach 39.4, angle 73
So the synapse is bigger.
Thanks for all your help and advice.
If you always thought it overrated, it does beg the question why buy it in the first place?
Kinesis Racelight 4S
Specialized Allez Elite (Frame/Forks for sale)
Specialized Crosstrail Comp Disk (For sale)
2020 Voodoo Marasa
2017 Cube Attain GTC Pro Disc 2016
2016 Voodoo Wazoo
I've never had a test ride of any bike that I have purchased in the last forty years. I know my fit on a bike, therefore , just take the risk, but it doesn't always work out. Saying that, I've bought a few frames and three of them made to measure which makes it difficult to test ride.
shaun668- Didn't mean to come across negative, wish you all the best with your new bike.
....or the difference is next to f**k all!
However I am the guy who cant tell one set of wheels from some 200gms lighter or my 105 shifters from my ultegra so you cant expect me to realise that my hands are 5mm higher and 3mm further away.