My guess is that you're using the standalone "lmg" tool (from the question). If so, this is obsolete and should not be used.
Instead, place an instance of the "mtline" component from analogLib, and choose the Type of Input as being "Field Solver". Then enter the physical parameters about the line in the property fields for mtline - you can still press "Display Cross Section" to visualize it.
The benefit of this approach is that the solver is run within the simulator, and generates the RLGC matrices (which are per-unit-length, and so do not have a length limitation). Because it's done in the simulator, you can parameterize the line's physical parameters, and sweep them, and so on.
With lmg, you could choose to produce either a lumped model or RLGC parameters. The lumped model had to know the physical length of the line, because otherwise it couldn't lump it - and this had limitations on the length of the line.
Since lmg has not been maintained, it will not have any of the improvements or fixes in the built-in field solver which is part of mtline. So you really should use that.