Resolution VS size

A friend took a digital photo with his camera best quality, tiff, and PS showed it had a 72 dpi resolution but a big size: about 70 x 60 cm. (I don't remember the pixels size but it looked high). I thought the resolution was very low for obtaining a good print but it can be printed with good quality at a 16 x 20 in size. Which is the equivalences between resolution, image size and pixels size? How you convert one mesure to another? Can I forget about resolution and care only about pixels size?

Thanks in advance.

Go to Image > Image Size, click off Resample Image and then change your resolution. The picture will get smaller, but you will get the resolution needed fpr print.

Spectra,

Solid information. The auto-resampling non-checkmark has brought a lot of light to my situation. I finally made a GREAT print for the first time last night.

Thank you.