question

You need the standard of the source: inter arrival time set by a statistical distribution for example the default function exponential. You can limit 100 generated items by stopping the source after the exit of 100 items through an OnExit trigger of the source.

The setting of an object size is a picklist option of the triggers: Visual > Set Rotation, Size or Location.

You add another picklist option in a trigger by the green cross icon.

Each coordinate value can be a discrete number or the result of mathematical term like a statistical distribution:

2 + (duniform(1,5,1)/10)  

This term creates numbers in the Range [2.1 ... 2.5] with a step of 0.1

The items have a randomly size, but there might be some of them having the same size. If you need unique random sizes you must create randomly value triplets that you don't used before. You put used combinations on a list, and if you don't find the new triplet in the list you use it to set the size of item exiting the source. Otherwise you build another triplet.