how big a starter??

I am only brewing from kits at the present hoping to move upwards in the new year. Trying to improve on what im doing and understanding that the yeast is a great factor , have built a stirplate and 2litre flask is on order.
When looking at the yeast starter calculator is asking for the OG (i know what it means) too determine the size of required starter, my question is how can i make a starter 48 hours in advance if i dont know what the OG is.
Im a little confused

Just use the planned og from the recipe.

Agreed, Use the estimated OG for calculating your starter. 

Keep in mind these calculators are used to Estimate the number of yeast cells produced.  The actual number of viable cells can vary by a factor of two from what is predicted fairly easily.  If you are making a starter from a slury keep in mind that the number of viable cells could be anywhere from 30%-99% even it was just harvested.  In addition, cell density can vary by a factor of 10 even if there is no viable difference.

More information on viability here:
http://woodlandbrew.blogspot.com/2012/11/counting-cells.html

Bottom line, you don't need to get the cell counts perfect.  Each liter of starter will produce about 100 billion cells, and each vial or smack pack of yeast is an additional 100 billion.

You're making it too complicated. Making a yeast starter is the best thing you can do to guarantee an active and aggressive fermentation. I've been using 2 cups of H2O + 3/4 cup DME + 1 vial of WL yeast for years. I put this on a stirplate a minimum of two days before brewing.

Yes Mike but how much wart will this do?
I make 50 litre baches so how much yeast do I need to pitch

Use the MrMalty pitch calculator

http://www.mrmalty.com/calc/calc.html