veanz, i know how you feel because i had alot of the concerns you have right now, i think i still do.
i haven't had a boyfriend in my whole 17 years, i feel fat sometimes and i also have strict asian parents! XD
you can eat what you like but beware of your health. too many of KKs aren't healthy for you, having them once in a while is fine.
i've never flirted/approached anyone but i don't think weight is such a big issue. if you are somewhat self conscious (and i think you are, i am too, hehe), naturally you're not going to ask someone who is too large or is too skinny because it'd feel awkard with them (unless of course you've known them for a long time and now suddenly fallen for them, which isn't what you're asking).
sure, i think some thin people are beautiful, but i get sick looking at anorexic models. stick figures aren't attractive, they can be as repulsive as looking at someone overly large.
as an average girl i'd say that most girls aren't thin (they're somewhat round or in the middle) so you can't expect much. some people are skinnier than others and we have people of all sizes and shapes. just be happy with how you look.
i try to be healthy but i often forget and eat too much at dinner, i suck at all sports so i never exercise. but recently i've lost weight and i think it was because i was sick. hahahaha. just don't worry about your weight too much and you'll be fine!
as i've never been in a relationship, i can't answer you last question, but whenever i go shopping with my friends, i usually tell them the truth. though no sane guy would ever tell a girl she's fat (unless he hates her).
hopefully my opinions on the issues above will help you!
good luck and don't be bogged down by the weighty issues!