My solution would simply be to change the variables that the bot uses for xpos and ypos to doubles. Then it wouldn't matter how big your sim went.
Internally the variables the program uses are Longs, so we are cool there. But the problem of course is how to project these values into the integer-valued array that the DNA operates upon. You know this of course, so what I beleve you are suggesting is to use additionaal memory locations to help when values exceed 32000 - an .xposbig and .yposbig such that a field position of (64001, 96005) would be represented as
.xpos 1
.ypos 5
.xposbig 2
.yposbig 3
This would not break existing bots that use .xpos and .ypos and would provide location precision for bots beyond (32000, 32000).
I'm okay with this. I'd also be okay with variants such as .xposbig and .yposbig counting in bot widths as Nums suggests and .xpos and .ypos wrapping at 32000. The exact x position of a bot would then be
.xposbig 120 mult .xpos 120 mod add
This latter suggestion has the advantage that .xposbig and .yposbig alone could be used for approximate position.