Best diet for my somewhat small cat?

From what I've been told by my vet, the best way to check is to feel the ribs and look at the cat from above (not the side since some cats have "floppy bellies", that isn't really fat).

Some cats are just larger or smaller framed, but you should be able to feel individual ribs, but notice a fat layer over them, and the chest should be larger than the waist from above, but not an overly narrow waist.

Sounds like you can be sure from the boniness that kitty is underweight though, so having quality dry available for free feeding sounds like a good idea. I'm personally a fan of Blue Buffalo Wilderness dry formula.

Generally speaking, wet food is healthier than dry, but that doesn't mean its better. Its expensive and quite often impractical, and leaving out uneaten portions is unhealthy, especially fish varieties, and can smell up the house. On a dry to dry basis though, you'll see wet usually has higher animal protein sources and less filler and veggies than kibble, and the moisture content ensures that kitty is well hydrated even if they aren't a big drinker.

Personally, with these actors I prefer a mixed dry and wet diet. I feed two meals of wet, and one meal of dry per day. Nice thing about dry is I can put it out right before leaving for work, and put it in food ball dispensers so they nibble on it slowly and it lasts them longer for the longer gap while I'm away. Some also believe that it can help slightly w/ dental health by firming the gums and allowing the jaw muscles to chew on occasion, and possible cause some scraping of the teeth plaque although you really need a specifically designed dental health kibble for that.

Has she been fully de-wormed? Parasites can really cause a cat to be too thin. A vet visit for every newly acquired cat is a good idea.

That does sound small for a Maine Coon. I would fill up the bowl with dry food every time you notice it's empty, and maintain a wet meal. That way, she can increase her food as she learns that there is a basically unlimited amount. My healthy looking adult cat gained 3 lbs over a couple months after coming home from the shelter, and I would call him fit-looking right now. Shelters are not the best places for cats to spend a long time, which is why their behavior and general habits tend to be so much different at a shelter as opposed to when you take the cat home for good. Right now she's like a guest in your home, and it's never polite for the guest to pig out. She'll come around as long as you leave ample food out for her and dote over her to show her that this is now her home as well.

FYI as a general rule, you always feed the amount recommended for the weight you want your cat to be (for an adult female maine coon, that's about 8 - 12 lbs) and that varies anywhere between 0.5 and 1 oz per lb.

Congratulations on becoming a cat owner! It's a wonderful thing to be. And welcome to TCS.

Generally-speaking, a diet of fresh raw animal-based foods is the healthiest diet to feed, although it's not something most people consider, or are even aware of, and it takes some self-educating to do properly.

After that is commercial raw and then commercial canned - specifically grain-free canned such as Wellness and Natural Balance sell at Petco, etc.

Kibble shouldn't even be marketed to cat owners, as it's an entirely species-inappropriate food source and can - and does - cause chronic dehydration (which stresses all the cats' organs and can lead to other complications), diabetes, obesity, pancreatitis, multiple urinary tract issues, and more.

It's a good thing your girl is already eating wet... if you decide to feed an all wet diet, you've got a head start.

What is your girl's name?

AC