Measure your bow from tip to tip. If it measures 54" then buy a 50" string. From there you can adjust your brace height a little by putting in or removing twists in the string. From the pic you showed I would guess you do need a 50" string. If the info on the bow said AMO 54" then you would have needed a 54" string.
Best and safest thing is to actually measure your bow length.
Here's a link on how to do it. https://asklancelot.com/how-to-determine-string-lenght/