The Personal plan gives you 5GBs of SSD storage.
For the Premium subscription plan you get 15GBs SSD storage.
The Agency plan gives you 50GBs SSD storage.
As for surpassing the given limits - you will be charged an extra $2 for each 5GB of extra website storage used at the end of the billing month.
For a more complete list of the pricing and advantages of each subscription plan, please see our pricing page.