Are “salad” and “pizza” uncountable nouns?

Hi. I was doing an exercise about countable and uncountable nouns, but I can’t tell if “pizza” and “salad” are countable or not. My book says that salad is uncountable and pizza is both, but it also says that the sentence “She’s having big salad in the café.” is wrong. Why? Also, what is the difference between using “some” alongside an uncountable noun and letting it alone? (For example in “I’m going to buy milk.” And “I’m going to buy some milk.”?)