To create custom properties just go to the top menu select
File - properties. This will popup a dialogue box that has 3 tabs, summary - custom - configuration specific.
Select the custom tab, in the last empty line you can type in a name in the "property name" column, select text in the "type" column. Then in the Value/Text Expression" column you can enter in a number or even easier click on the extruded length dimension of the model (to do this you can double click on the model and the model properties should be displayed) select the appropriate dimension and your done. Make sure you select the done button at the bottom or your changes will not be recorded. create a drawing of the assembly, insert a BOM the click on the BOM, on the left side of your screen a dialogue box will show a list of editable items for your BOM, select Colum properties, select the custom property button, this should have a list of all the available custom properties from your model. Select the one that matches your name and you should be able to do the rest. Before you modify a default BOM you may want to save it as a template for a new custom BOM with your specific mane for it.
This was based on how SW2007 looks.
SW 2007 SP 2.2
Dell Inc. Precision WorkStation 670
2048 Megabytes Installed Memory
NVIDIA Quadro FX 1400
Windows XP PRO SP 2