Define the duration bij e.g. Datediff("minute", [start time], [end time]) and drop this on the size tile in the marks card
You have a field called Duration - though it looks to be defaulting as a "date field". Is this truly a date, or is it number of days? If it is number of days, then I would change the default data type to Number, and use it on size, as Norbert suggests. Or you can date diff "issue start, and issue end". Of course, this assumes that all your issues are closed/resolved. For open issues, you might have to use "Today() where Issue end is null. " For that you can try an IIF calculation to return the end date, and wrap the datediff around it..
More on Calcs here: Date Calculations