Mm yeah, my stomach always feels terrible when I eat them unless I eat a lot of food... you get weird shits the next day too haha. A 20bag (1.8ish grams depending on the dealer) of decent shrooms will do you fine though. You'll still see cool visuals, think cool thoughts, and feel great.
In fact, I'd go with a 20 before you try a full slice, just because shrooms are different from things you've probably tried before like weed in that you're not in complete control over your thoughts and emotions, and that can be scary if you don't know how to handle it. I took them the first time and had what was possibly the best experience of my life, thought I knew everything I needed to know for the next time and realized just how powerful a drug it is when I sat in the fetal position crying for an hour because I was so worried about my life and my future and what was happening to me.. scary shit, especially because I'm a very laidback person who rarely ever worries about shit like that. Don't mean to scare you haha, just play it safe, do it with friends and in a good mindset, and you'll have an incredible time.