so a 4x8 beam would be to big?
The question is not correct
When you are talking about I-beams you have to specify the type. Because regarding the shape, the strenghness does change. So if you have the same measurements at a W-, S-, C-, MC-beam, or a german kind, they are all different. The steel is thicker or thinner, it changes the thickness to edge or not, and so on.
Additional to the shape the question is also how "hard" is the steel, the 'ksi' value.
There is a reason why these books of steel are so big.
I understand that a lot of friends here in this forum hear that theory-stuff and calculation the first time. They have a hugh knowledge and know how to work with their hands, but have never faced such need's to do such mechanic calculations. You have such stuff at collage or university when you study mechanics, constructions or like me medical engineering. But a guy who study computer, business or agricultur or running a farm, a workshop or a landscaping business will have to make extra classes...
@ skeets: So what is your question again? Or better, do you also want to place somewhere an I-beam?
der carl