Well rule 1 is if you're a woman shopping in the USA you cannot go into a store and buy a piece of clothing without trying it on and expect it to fit. It might fit in the same way the ball might land on the number you bet on at the roulette table, but there's just as much chance it won't fit even if the size on the tag is the exact same size as the clothes you're currently wearing. This is because there is no standardization of sizes, and there's this thing called "vanity sizing".

Outside of that, different styles of clothing will look different when worn depending on the person's body shape, and the clothing can also make your body look different to a certain degree by emphasizing or de-emphasizing parts. That's an example of what your daughter said about the baggy pants making you look bigger.
When it comes to discovering what is or isn't flattering on your specific body you really just have to try things on and see.
The same is true for what colors look good on you.
There's really no trick to it other than that.

Also, pick and choose which trends you follow. Just because rompers are 'in' doesn't mean every person should be wearing one (in fact, in my opinion no one over the age of 5 should be wearing a romper!).
That's not to say you want to be wearing leg warmers in 2015 because they supposedly look so great on you, but simple styles can work in any era. For example, jeans. Yeah, skinny jeans have been all the rage for a few years now, but they look awful on me I usually go for bootcut instead.