Sometimes there's confusion about which bonnet size to choose. Here's a brief overview on bonnet sizing as it relates to rotary and orbital machines. Also a breakdown of pad driver sizing as it relates to the machine size.
Pad drivers generally run 2 inches smaller than the deck size of the machine. That means if you have a 17" machine, you'll actually have a 15" pad driver. If you have a 19" machine, you'll actually have a 17" pad driver.
With any machine, you'll ALWAYS want to have some overlap with the bonnets - beyond the edge of the pad driver. For example, if you have a 17" rotary machine, at the very minimum, you'd want to have a 17" bonnet. Keep in mind, the 17" machine actually has a 15" pad driver, so with a 17" bonnet you'd be adding an additional inch (1 inch) of bonnet around the perimeter of the driver.
Now let's bring this into perspective with an orbital machine. Orbital machines are highly jiggly as you know. So it becomes even more important to run the bonnets on the large-ish side. In the case of an orbital machine it's recommended to go 2 inches larger than the deck size of the machine. In other words, if the deck size is 19", then ideally you'd want to go with a 21" bonnet.
In the case of a 19" orbital machine you're actually running a 17" pad driver. Therefore you'd need a 19" bonnet at the very minimum to sufficiently cover the driver. Recall that a 19" bonnet will add an additional inch (1 inch) of bonnet around the perimeter of the pad driver. But an extra inch doesn't amount to very much wiggle room on an orbital machine. So with a 19" orbital machine, you'd really want to consider going with a 21" bonnet. And with a 17" orbital machine you'd likewise want to increase 2 inches and go with a 19" bonnet.
Bottom line rule of thumb: For a rotary machine, select a bonnet that equals the deck size of the machine (actually 2 inches larger than the pad driver dimension). For an orbital machine, select a bonnet that's 2 inches larger than the deck size of the machine (actually 4 inches larger than the pad driver dimension).
Selecting the correct bonnet size will improve the performance of the machine (especially in the case of orbital machines). And you'll enjoy working with the machine much more!