Can anyone help? I cannot figure out what breed she is for the life of me.

My cat is a large cat, she's not fat, just very massive, she's larger than my domestic short hairs. even her paws are so large... 

She is beautiful ! I love her tall ears and extra long whiskers

Her colour and pattern is a brown mackerel tabby. She is a domestic shorthair, domestics do come in all shapes and sizes. You can get very tiny ones up to huge ones like yours that's the great thing about domestics such a great deal of variety, you never know what you will end up with :lol3:

I think she might have a little Maine Coon in her, given the long whiskers, the characteristic M above her eyes and her size. How old is she? How much does she weigh? Does she have little tufts of fur in her paws?

I would say she has Maine Coon too. Beautiful classic tabby!

The "M" on the forehead is not restricted just to the maine coon breed. It's part of the tabby pattern which is found in all breeds and domestics. That would be like saying all blue/grey cats are Russian blues. Plus you get solid coloured maine coons which would not have this M due to them not having the tabby pattern.

I own and breed maine coons and I don't see any maine coon in your girl. She will not have tuffs between her toes as she is short coated only long haired cats have tuffs between the toes.

A very beautiful domestic shorthair

She's a rescue.. Maybe 3 years old.  She weighs about 20ish pounds (most is like, muscle something, because it really isn't all fat). I am in love with her no matter what she is. But it would be nice knowning what I'm dealing with. 

The pic on nr 2 looks quite unusual for a common domestic.   It makes me think on inbreed from some wild cats..    And she is big too.  A descendand from some of the new breeds with semi-big african cats mixes?

A well-fed cat can grow to be pretty big in adulthood--and I'm not talking fat, I'm talking long and tall and strong. Like, Tiny, whom I rescued as a kitten--he weighs probably five pounds more than Christy, who was rescued as an adult after a relatively hard life. Males usually weigh more than females, of course, but not usually five pounds more. Tiny got the chance to have some of his growth in a good environment where he had all he wanted to eat and didn't have to deal with parasites (his name was given to him by the neighbors' kids, and needless to say it doesn't fit him in the least.)

Kittens raised from the beginning in good homes may grow even bigger, if their genes allow it. Some cats stay small because they're naturally small, but many cats are small simply because there wasn't enough food for them to grow big. Twenty pounds is on the very high end of normal, non-overweight cats... but it's not out of the question! As best I can guess, you just ended up with one of the hefty linebackers of the cat world.