Shopify does lots of things well, and one thing in particular is the ability to scale. To give the engineers a level of control there's some limits on certain things - page size being one.
"But 64kb is so small dude..."
No, it's really not. If you've got a website that is punching out a page size greater than this you might want to go back an revisit the html in place. The few times I've seen Shopify stores with this kind of crazy size is when there is very complex <table> structures in place. Now if that table was also copied from Word the size is going to be even bigger since that tends to come with a pile of inline styles attached as well.
Clean up your code.
If you get this error just pause, and review the code in place. Double check that your not adding insane tables of blocks of HTML that can't be handled with something simpler.
"I needz all the codez. I can not clean it up..."