If you load custom shapes, the recommended size is 32x32pixel.
^Bottom of this screen says the below:
Suggested Size - Unless you plan on using Size to make the shapes really large, you should try to make your original shape size close to 32 pixels by 32 pixels. However, the original size is dependent on the range of sizes you want available in Tableau. You can resize the shapes in Tableau by clicking Size on the Marks card, or by using the cell size options on the Format menu.