Emphasizing size:
* As big as a house.
* Like a towering redwood.
* As large as a mountain.
* Like a skyscraper reaching for the sky.
Emphasizing strength:
* As strong as an ox.
* Like a raging storm.
* As powerful as an earthquake.
* Like a force of nature.
Emphasizing presence:
* As imposing as a fortress.
* Like a shadow looming over the land.
* As awe-inspiring as a cathedral.
* Like a beacon in the night.
The best simile for you will depend on the specific context and what aspect of "giant" you want to emphasize.