I'm planning on getting a new road bike, and I know most bikes' measures are C-T, and I don't know why people keep asking for the top tube measures. Although I've tried several bikes, it's been hard for me to determine whether me being comfortable or rather it's the way it should be. I'd say it's because I'm inexperienced and unable to deduce, so I thought I would seek some professional help. My question remains, How do I know what top tube length suits me best? I'm 5'7, and the bikes I tested had a C-T length of 52cm and 54cm, so I assume they're medium sized frames. Thanks!