28 inch waist is a medium in their chart. I found the "Loose Tights" to run small (I usually wear a L) so if I order these I'll go for XL.
Medium might work for you, especially if you are thinking of wearing it over your bike shorts. Call them on Monday and ask what they think, they are very nice over the phone about helping with fit.
My heart sank when I saw they only had M, L, and XL left; but there is still hope!
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson