The OrgChart software defaults to automatic (Auto Adjust Size) box sizing. This feature ensures that all boxes are always being calculated to be the optimal size. However, if you want to override automatic formatting and make boxes a particular size, you can certainly do this. You can change the size of each box manually, but that can be very tedious. Another option is to make one box sized manually, and then use the format painter to propagate that size to other boxes in the chart. But note that even if you make a manually sized box, that particular size may not persist from one page to another, because some pages may have few boxes, and some pages may have many boxes.
Whether you use manual or automatic sizing this will be the case you can use the Format Painter wizard to make boxes a comparable size. If you want to perform manual sizing this article may help.
Resize Boxes: Select new box size
To get all the boxes one size select a box that is the size you want for all boxes.

Go to the Home tab, click on the Format painter wizard.

Resize Boxes: Select format
Go into options and click on the manual check box for the Size. Uncheck every other check box, and apply it to the entire chart on the right hand size and click OK.

Resize Boxes: Display results
This will automatically adjust the box sizes for you throughout the OrgChart.
