Just one of those silly things - I like my Roxterm window to pop up centre screen when I hit Alt c.

Try the command roxterm --geometry 80x25+500+300 and see where it appears.
The 80x25 sets the size and the +500+300 sets the x and y coordinates.

These values are guidelines only and based on a 1680x1050 laptop screen:-
screen #0:
  dimensions:    1680x1050 pixels (444x277 millimeters)
  resolution:    96x96 dots per inch
  depths (7):    24, 1, 4, 8, 15, 16, 32
  root window id:    0x275
  depth of root window:    24 planes
  number of colormaps:    minimum 1, maximum 1
  default colormap:    0x20
  default number of colormap cells:    256
  preallocated pixels:    black 0, white 16777215
  options:    backing-store WHEN MAPPED, save-unders NO
  largest cursor:    64x64

- other monitors are available and will result in these values producing different results - you may need to use trial and error, like what I did, to get the best results.

When you're happy just update the values in Obkey, save and away you go.