I’m afraid it is not possible to change the size of the color bubbles in any chart legend - in a grouped map, the legend will display the group names and their color, you cannot group them at the same time by size and color. By default, an icon map chart will automatically adjust the size of the icon depending on the value you have entered inside the map’s data table (in the Value column), and it will group the entries by color, depending on the group name you enter inside the Group column in your map. Hence, I would suggest simply filling in your data inside the data table like that - values inside the value column and group names inside the Group column, and everything should work!
Here are detailed instructions on Placing markers on a map in Prezi Design. Please, let me know if you have any additional questions!