I use a pea-size gravel in my tank. Rather take something that isn't too big, since it could get stuck in their mouths, but don't get anything too small either since they could swallow it. So something in between
Sand isn't suitable for goldfish either due to the small particles and how they dig in the gravel for food. They may get digestive problems from the sand. As for the plants, live plants look better, and they're safer since they don't have any sharp edges like some plastic plants
I have successfully had various types of Anubias, Java Fern and some Java Moss in my goldfish tank, very hardy and fast-growing plants that they won't eat.
Goldfish, depending on the type range from 6-12 inches! Where the celestials and bubble eyes are the smallest (and hardest to keep), in between sizes are fantails, ryukins, telescope eyes and pom poms. Orandas grow the biggest
But if you want to keep fancy goldfish, you need to make sure to give them the correct foods. A mix of vegetables, live foods and pellets. There's a sticky somewhere on the forum about "how to feed veggies to goldfish" or a similar title. There's also mentioned all the other supplements they need to be healthy and avoid digestive problems and swim bladder disorders.
Plecos should generally not be kept with goldfish unless you know for sure what kind of pleco it is, and if it has the same water quality requirements as goldfish do. There have also been cases where pelcos have sucked the slime coat off the fish, but... I've kept suckermouth catfish of various kinds with my goldies for years without problems. But do some research and see which types of goldfish/plecos would fit you!!