Assuming you have selected the group that contains all the sections in that model and then right clicked and selected "explode", you should be able to select each individual section group and use the scale tool to resize each one. You can use a bit of math before using the scale tool to work out your desired dimension and after dragging on of the scale axis cubes, tap the ratio you need to use to get the right dimension into the "scale" entry box on the lower left of the Sketchup window. You will likely have to play around with which scaling cube to use dependant on the axis you want to scale within and to be honest for H sections its probably quicker just to draw your own as a 2D shape and then extrude it as some of the scaling will alter the thickness of the plates making up the section as well as the overall height/length (see next sentence).

If its the length you are altering then after exploding the main group and deleting all the unwanted sections, double click the group/section you have left until you can select to end/cross section face and use the push/pull tool to lengthen or shorten the section (again you can enter the length to lengthen or shorten in the entry box).

Oh another handy tip if you want more precision than you get by default is to go to Window - Model Info - Units and in there you can increase the precision. I use 0.001mm normally for component work although I doubt I'll be able to get that on any mchine I make ;-)


Ooops I was too slow ;-)
Hope that helps, but if you're really stuck let me know the dimensions you're after and I'll whip sopmething up and share in 3D warehouse.

Cheers


Chris