I'm not the most fashion conscious person, I will admit that. 75% of my wardrobe is graphic tee-shirts and color coordinating is an alien concept to me (being extremely mildly color blind does not help. For the record, I can see colors just fine, it's differentiating shades of them I have a problem with).

trying to figure out sizes gives me a headache, never mind that in general, no two brand's clothing sizes are exactly the same... (this applies to mens clothes as well. no two are exactly alike.

The really rough part is when you're in the closet about your gender and you don't want to go wandering into a store and talk to a person.

So, here's a few things you should know.

Dress size.

To figure your dress size, you need three measurements. Your bust, your waist and your hips. How those get converted into a size number is beyond me (I was never good a math ether, but that's nether here nor there). 

Bust measurements should be taken just below the arm pits, whether or not you'll need to wear a bra while doing this depends on the dress. 

https://www.catrinasbridal.co.uk/dress-size-calculator/

https://www.annascholz.com/customer-service/size-guide

(for the plus sized girl especially)

Shoe sizes are a bit easier.

Take your US shoe size (many shoes, especially the cheaper ones for a place like Walmart, list multiple sizes on the inside) and then just add 2.

So, for example, everyone in my family has big feet with long, almost finger like toes. I take a size 12W/13 in men's shoes, which means I'd be looking for about a size 14 or 15 when I buy my first set of heels.