unfortunately, this didnt work. I tried placing them at positions that were multiples of 4 and making them sizes that were multiples of 4, and saving but when I open the level up later it they still get re-sized. (always to the same positions)
I'm not sure about the repositionings, but steel areas can be no larger than 64x64. If you're still having trouble with steel areas, attach the level here, and let me know where you try to place the steel areas and where they actually end up.
I just noticed that some other regular terrain pieces go missing as well.
There are limits to the range of x and y values supported. For example x cannot be less than -16*.
This page has some more details. (Note: technically they are for WinLemm, but should be close enough for the most part even for DOS Lemmings.) Lemmix's UI doesn't quite properly constrain you from using bad values, but you can run the "Validate Level" command under the "Tools" menu to find out things that may be out of range or otherwise bad.
If you don't think that's the problem, let me know what terrain pieces you are using and where you are attempting to place them. The "Inspector Window" (F11) will show these details for the current level element you selected.
*What I said earlier today in IRC is wrong. However, I'm sure I had previously run into a different Lemmix bug related to negative values.