Tony Stark thinks May Parker is an attractive young woman, but when he hears the word "aunt" he thinks of an older woman that isn't sexually attractive. In English, the phrase "all shapes and sizes" means that you shouldn't assume the shape and size of something, because sometimes you will find an example that is beyond your expectations.
I'm from Canada and we have larger cars and trucks. When I visited Japan I noticed many tiny cars and trucks. If I said "I'm surprised at how small those trucks are", you could respond "don't be surprised, trucks come in all shapes and sizes". This means "don't think that all the trucks in the world are going to be like the trucks you have in your home country, different places make different design choices".
Tony Stark thinks that all aunts are old and unattractive. May Parker says "don't think that all aunts look like the negative picture you have in your imagination, we come in ALL shapes and sizes".