If the lifters are the same as the OEM hydraulic rollers, you'll be in the 7.2" range. IIRC most others are in the 7.3" range. But Crane uses very short p-rods for some of their hydraulic roller lifters and cams (just over 7.00").
If you know the brand of the lifters you can research it from that, or by measuring the distance between the p-rod body and rocker arm, then adding the depths of the cups. This will get you a ballpark figure to work with to make a tool.
Other things that can effect p-rod length is the valve length and cam base circle diameter, so you will need to mock it up once you get a ballpark figure to work from. A couple pages that will help:
• Adjusting hydraulic lifters
• Valve train points to check
• Valve train geometry
• Valve spring installed height
Click on image for details on making an adjustable pushrod:
Other homemade pushrod length checking tools at HomemadeTools.net