I just ran across this last night. Obviously, most of the time one can export at a precise resolution using the resize options in the export dialogue box. However, sometimes one wants a resolution which isn't precisely the given aspect ratio -- eg, I was trying to export for a desktop background at 1366x768 last night, which isn't precisely 16x9. I just couldn't get LR to do it. It kept exporting at 1365x768, rounding the other direction. Is there some way to force it to a particular resolution? I even tried recropping manually entering the ratio as 1.366 by .768, but it decided I really meant to say 16x9 and just reverted to that.
Ultimately, I just exported at full resolution and resized in a different program, but I'd like to know if there's a way to do it LR.
Thanks for your help,