So if I understand you correctly, you have the different sizes of blocks, but you want to access these different size plates? First of all this ISN'T a super-easy process and I can describe one process.
Double click on the existing block, Edit Block Definition dialog box comes up. make sure your block is highlighted. Also hover your cursor over these new tools that open up. Make sure Block Authoring palette is up.
1)Enter one plate at 0,0 and select explode block in the dynamic block editer. 2)On the paramters tab, click linear parameter. 3) Click on the center of the plate (onsnaps help) 4) Click on the outer edge of the plate. 5)Pull up to and left click to place the distance dimension.
You will see an exclamation point. This means you need to place information in the block to work.
6)Go to the actions tab. 7)click on Scale and it asks to select parameter (which is the distance parameter you added). 8) Select objects which is everything. 9)left click somewhere outside of the block to place the lightning bolt.
9)Left click on the distance dimension and then right click and choose properties. 10) Scroll to the bottom and the two heading in the properties palette you are looking for is "Value Set" and "Misc". 11)Under Value Set, click in the Dist type field and change thru a pulldown to List. 12) Now click in the field below called Dist Value list. 13) You will see three small dots to the right of tht blank field, click on the small button. 14)Add distance dialog box comes up. Enter a number in and click Add. 15) When you are done adding numbers, click Ok. 16) Go down to the next heading called "Misc" and change number of grips to 1 insted of 2.
If you entered several diameters for the plate, click Close block editor button just below the typical CAD toolbars. Save. Now left click on the block so that the grips appear. Left click on the grip and drag it a bit, now type a number which is one of the pre-defined numbers you entered in the properties palette. It will automatically scale.
There is another process, if this isnt what you are after I can tell you of the other.