Linear addressing is hardwired to 14 Mbytes for ISA boards and 78 Mbytes for VLB boards, thus if you have more than that on your board you shouldn't enable it. The aperture is selected from the VideoRam parameter of the XF86Config or from the amount of memory that is detected if VideoRam is not found.
I hope (because I have not tested it very thoroughly) that linear addressing will work on all ISA boards, VLB ones work flawlessly.
$XFree86: xc/programs/Xserver/hw/xfree86/doc/sgml/Oak.sgml,v 3.12.2.4 1999/08/18 13:12:11 hohndel Exp $ $XConsortium: Oak.sgml /main/8 1996/05/12 20:58:00 kaleb $