@Tamer ElNashar -

select them all and click on the size option in the toolbar - it will let you resize them to Small, Medium or Large (preset sizes).

In addition to @Kiron Bondale’s perfectly correct answer, you can select them, then drag a corner of the selection to make them all smaller or larger compared to how they are. So for instance if you wanted a bunch of different sized stickies to all be the same size, twice as large as the L preset, then you can do what Kiron says above, then drag the corner to make them all larger together.

How can you match the size of one batch of custom-sized stickies to another batch of custom-sized stickies? When I do this, only the bounding box aligns with other objects. I don’t get the same dimension match indicators as when I scale stickies independently. 

@gregory fischer - You could take one of the stickies that you want to match the size of, then select your other group of stickies that you want resize, use the auto-layout tool to align them, drag them beside your target sticky until the tops are horizontally aligned, and then drag the group of stickies until the bottoms are aligned too. And because all of that was likely too much to just magically visualize in your head, here is an example:

How can you change notes into the same aspect ratio, I have not found a way. 

You can use the switch type action:

In action:

@Robert Johnson thnx