I want to bake a sheet cake for a friends birthday. I am planning a white cake with a stabilized whipped cream filling with a layer of pureed berries as well.  Planning to use a cream cheese frosting. What size cake is 1/4 sheet cake and will it feed  20 people? Also I am concerned over torting the layers if I need to go to 1/2 sheet cake. Suggestions would be so appreciated.

Sizes of sheet cakes vary depending on where you look.  It's better to just use the Wilton chart and go by the number of servings.  According to Wilton based on a 4" tall cake.  (For a 2" tall cake you would refigure for a 2" x 2" piece.)

 7 x 11 serves 24 party piece; 32 wedding

9 x 13 serves 36 partypiece; 50 wedding

11 x15 serves 54 party piece; 74 wedding

12 x 18 serves 72 party piece; 98 wedding

A 1/4  sheet cake is generally 9x12 or 8x13 depending on your pan.  It will feed 20 people easily depending how big you cut the pieces. I have the Wilton 1/2 sheet pan which, I believe, is 12x18 so I get a 9x12 1/4 sheet which is approx.  25 servings (based on a 2 layer cake).

A recipe for 1- 8" cake (2 pans) will give you about 1 layer in a 1/2 sheet pan.  So, if you have a 1/2 sheet pan just make one recipe and cut it in half and stack.  

If you need a 1/2 sheet, just make 2- 1/2 sheet cakes and stack.  No need to worry about torting :)

Hope that helps you:)

Oops, pastrybag lady types faster than me.  Took me a bit to fix all my typos, haha

Thanks to both of you.  I plan on using the doctored cake mix recipe for a moist cake and a whipped  cream filling with cream cheese added to make it stable and a thin layer of strawberry puree over this. Will that cake recipe hold up to stacking in a sheet cake without problems? Should I  also use the filling to ice the cake or change to a cream cheese buttercream  for the icing?

Whether  using cream cheese is a good idea is dependent on the temperature of the venue and the recipe you use.  Cream cheese has about a 4 hour shelf life in warm temperatures with the typical recipe.  If you use the shelf stable recipe which is roughly double the sugar the cake will last longer but be super sweet.  Either recipe doesn't hold up well to high temps.