Are there any lower or upper limits to how much I can create a savings?

As long as there is enough Available Token Quota for the day on the particular token, you are free to place as large a savings as possible.

However, there is Minimum Savings amount showing on the "New Savings" prompt in Token list of Account page. This is set depending on the network fees of each tokens. The Minimum Savings amount and the minimum transfer amount for each token is the same and it is there to prevent your tokens being wasted on network fees.