I'm not sure how mixing creature variations will work out for you, since some tags may overwrite each other, but ANIMAL_PERSON will gravitate a creature's size up to 70k, whereas the GIANT variation will double it's size. If it works the way I think it will, the creature's final size will be 140,000.
As for making the creature stronger without toying with attributes and size, you can give it's natural attacks a higher velocity modifier to increase the force going into them. Though this won't affect things changed by strength, such as carry weight and muscle thickness.
The 5000 cap is only for starting stats, creatures can go beyond that under the right conditions, I believe.
You can make the creature increase it's strength faster by using [PHYS_ATT_RATES:STRENGTH:500:3:4:3] The numbers I put in there are the default values. The first, 500, is the "cost to improve" modifier, and a lower number will increase an attribute faster than higher ones. Though 0 and negative numbers will not work.
The other 3 are for attribute decay caused by not using an attribute over a period of time. Setting them to 0 will stop attribute decay entirely.
I found
[CV_NEW_TAG:CHANGE_BODY_SIZE_PERC:200]
in the Giant variation, but the wiki gives a whole range of size multiplications for giant creatures, Giant Lions are double size, but Giant Sperm Whales are quadruple size and Giant Lynx is roughly 15 times as large. Either there's some complicated interactions going on, the wiki is incorrect, or that's a really complicated mathematical operation of varying contour.
I also found a whole bunch of stuff that was going to mess up what I was going to try and do.