The problem with a bigger goliath is that it would probably require a rebalance in stats to make it feel right.
The current goliath has 1.76x times more HP than the reaper. Meaning, assuming same density, the goliath should be on average 1.2x longer, wider and taller. That seems to be similar to the current differences in size. Assuming the goliath is made of stronger and heavier materials, the difference should be even smaller than 1.2x. If its dimensions would be increased a lot, current HP pool would be disproportionally low for it and it would feel plasticy - every time you would use it, it would lose HP faster than its dimensions would indicate.
However, I really like the idea of a strider tank. Something bigger than the goliath. There are definitely problems with the concept and it has been discussed before, but if there are enough people supporting the idea and somebody with the artistic skills and willpower to make a proper model for it, I don't see why it shouldn't be discussed/tried. Anyway, that's a topic for another thread.