From my experience with other Yacht Dice (like Nintendo's Club House on the Switch), a Small Straight is usually 4 dice in a numerical sequence (so for example 2-3-4-5), while a Large Straight is 5 dice in a numerical sequence (so 1-2-3-4-5 or 2-3-4-5-6).
I guess it depends on the version though! The Wiki for Yahtzee has the same rule as the one from Club House, but I did see a board game forum's page for Yacht Dice saying that a S Straight is 1-2-3-4-5 and a L Straight is 2-3-4-5-6 (it doesn't seem like the most prevalent version, but it does exist).