even if they do think from your stats that you are overweight, it's not going to be some big deal to them-- i think you are more conscious of it than they will be really-- they are used to big-big guys going in after all with MUCH more weight than you have, I'm sure-- and they will NOT be critical that you plan on doing something about it ie. cycling to get fitter-- you will be respected i reckon for that alone...

most people i see in my LBS are just regular looking folks of all shapes and sizes like you and me and not super-athlete-types at all.

imho, i would fill in the weight box online-- you will know if you are getting a disrespectful vibe or not when you get there-- i strongly suspect though that they will be so pleased to help you find something right for you-- LBS are in the business of turning people onto cycling and encouraging them to get out and do it, so dont worry about your stats-- just go and enjoy the test rides and HAVE FUN!

and welcome to TE :-)
batsheva