The right size is important, because baking times are designed for a specific batter thickness. If a recipe is meant for a 26 cm pan and you pour the whole batter in a 18 cm pan, it will be much thicker and the middle won't get done before the top burns. That is why high cakes are made by stacking layers, not by baking one high cake.
You can pour a batch of batter into multiple small cups and have it turn out all right as long as the height stays more or less the same as it would have been in the large pan. My advice is to calculate the area of the pan given in the recipe and the area of your silicone cups, so you can know how many cups to use. You don't have to be completely precise, for the average cake it is not too terrible if you have +-0.5 cm difference in batter height. If the difference is larger, you can still get good results, but you should adjust the baking time.
The resulting cupcakes will still have the thickness of the normal cake, or somewhat more if you use less cups and bake accordingly longer. I would advise against trying to bake high-rise muffins from normal cake recipes. Even a sponge cake recipe might need some adjustment of the amount of baking powder before working as a high muffin. Other types of cake, especially the more exotic ones (genoise, flourless cakes) are unlikely to bake well if you fill them into deep cups.