Problem

When creating Sheet Models in MicroStation, the print dialog should automatically populate with any paper size defined in the Sheet Model itself (i.e. the name in the "Size" dropdown list under "Display Sheet Layout"). Typically this problem manifests when using the system print driver (printer.pltcfg) which is accessing custom form sizes for the print queue being used. Even though you've matched the Sheet Model form (paper size) and the print queue form names, there is one additional step which needs to be done.

Area: sheet model, how to

Solution

From the installation folder of MicroStation, copy the file: C:\Program Files\Bentley\...\MicroStation\Default\sheetsizes.def to a location in which you have stored your organizational files e.g. C:\ProgramData\Bentley\...\Configuration\Organization\Data\ (also consider to rename the file, so that it reflects your organisational name).

Edit that sheetsizes.def file (text editor) and add the custom form name(s). 
Make sure the variable: MS_CUSTOMSHEETSIZEDEF points to the correct location and name of the sheetsizes.def file.
After restarting MicroStation, the new sheet size(s) will be available for selecting.