Matching tire width to rim size.

[Sxphn]

How can I know the largest width tire my rims will safely accept for touring with a load?  My recumbent bike came to me with a 28 mm tire on the rear wheel and at the builder's website it list the rear wheel as 28mm.  It seems I read somewhere that you could only safely move up or down one tire size which suggests a 32 mm is as large as I could go.  However, I no longer remember my source so I am relunctant to trust what I think I read.  Can anyone advise?

[OmahaNeb]

Sheldon Brown's web sit has great information for how wide a tire will fit on a rim.  Use this link and at the bottom of the page is a chart.
http://www.sheldonbrown.com/tire_sizing.html

[Sxphn]

OmahaNeb:  Thanks for your reply.  I found the exact information I was looking for at the link you supplied.  Now all I need to do is to remove my tire and measure the inside of my rim.  I am glad I asked my question in the right place.  Thanks.