oadie wrote:
It's nice to have a range of thicknesses at the ready, if you can afford to (that applies to sizes of rod, tube, and architectural shapes, as well). For general work and detailing larger pieces, .020" and .040" (1mm) are quite handy. I also use quite a bit of .030" (approximately), since that's the thickness of the styrene signs I found at the hardware store - low quality material (flexes quite easily and has a slight texture), but it's cheaper than the HIPS sold to modelers and takes solvent cement just as well.

For the main structural elements of a project the size of a titan, you'll want something relatively stiff - anything thicker than 1mm would do nicely, although you'll still need to rely on internal bracing and/or lamination to keep the sheets from flexing over large voids unless they're quite thick. In those cases, you're often better off building the basic shape from foamcore and skinning it with styrene.

 Sean_OBrien wrote:
Another thing to keep in mind is stuff other than Styrene...

For larger structures - styrene is fine, but I actually prefer cellular PVC (sometimes called expanded PVC). It is much cheaper and lighter than styrene. Easy to cut, drill and carve. You can glue it to other PVC using PVC cement (very handy when dealing with things like titans than often have PVC hip, leg and arm joints) and you can glue styrene to it using superglue or special plastic solvents (Plastruct makes one which will work...think Ambroid does as well...).

http://www.usplastic.com/catalog/item.aspx?itemid=23330&catid=733

For titans and buildings - I much prefer the thickness to styrene. It lets me provide the necessary bulk for masonry and cement walls in a single layer as opposed to having to glue up several layers or have that paper thin wall look that many things have when just a single layer.

For styrene - don't forget thinner things as well. The 0.005" sheet comes in handy when you want to replicate things like cloth. You can cut strips and use them for straps for cargo, or flags/banners. It isn't rigid though at that thickness, but you can shape it.