This should work:

forever
set size to (((-2.5) * ([abs v] of ((percentDistance) - (50)))) + (200)) %
end

Here's a graph on what it should do to the size based on distance:

You can get your percent distance by dividing the total distance by the current amount traveled and multiplying by 100:

set [percentDistance v] to (((totalDistance) / (traveledDistance)) * (100))