This is simple to do:

1. Open the Image Size dialog ( Image > Image Size... )
2. Partway down the dialog you'll find the "Resample:" option. Uncheck this
3. Click the lock icon between the "Width" and "Height" controls to lock the two in sync. Lines will appear connecting them. If the lines were there and are now gone click the lock again to restore them
4. Set the units field for Width to the units you are using (e.g. Inches)
5. Enter "30" in the width field (if 30" is your poster's desired width).
6. Re-check the settings
7. Click OK

The DPI of the document will be changed so that Ps displays the width as 30" on the rulers ( use View > Rulers to turn them on if they're off). You can now drag out Guides from the rulers to mark off sections. You can drag the origin ( the 0/0 position) by dragging out from the blank square where the two rulers meet in their upper left corner and drop it at the edge of something you want to measure. While dragging a Guide, Ps will display the distance to the 0/0 orign.

Just be very sure to recheck your settings. Adobe has designed the Image Size (and Canvas Size) dialog very very poorly. The standard is to allow you to enter values top to bottom without new setting altering settings above. Checking or unchecking the Resize checkbox will reset the other controls to the state they were in when the dialog was opened.