I second Frank's recommendations for test strips.

And with large prints, you might want to consider viewing distance. All small prints get viewed up close, but some large prints will only get viewed from far away (unless you're printing for the handful of jackasses like me who might actually stand on the back of the couch for a closer look).

The ideal sharpening scheme will be dependent on your printer/paper combination, the nature of the image, the nature of the scan, and the viewing distance.

When you find a formula that works, you'll probably get to the point where you can judge on screen for other similar images, that are meant to be printed and viewed in a similar way. But until you get to that point, printing test strips will be the most reliable.