At the very least you will need 1lb of strawberries/gallon of beer, if not 2/ gallon. At, the very least 1lb/gallons for a subtle flavor. You may no be able to pick up 1lb of strawberries in 5 gallons? Strawberry is a pretty subtle fruit when used in beer. A club member of ours a few years ago made a strawberry kolsch with fresh ripe pureed strawberries. It was fantastic, maybe the best fruit beer I have ever tasted. I believed he used 2lbs of strawberries/gallon, if not more? Designing Great Beers by Ray Daniels is a great resource for fruit beers.
EDIT, fixed my ratio of pounds of fruit/gallons of beer.