As accordance to the Payment Services Act (PSA), you'll be able to hold up to S$5,000 (or equivalent in other currencies at the end of the day Singapore time) in your account. This also applies to overseas ATM withdrawals.
There are no daily transaction limits. However, do note that the annual limit for transactions (including overseas ATM withdrawals) is S$30,000 (or equivalent in other currencies). This annual limit is calculated every 365 days, starting from your first top-up on or after 27 February 2020.