Calculate the volume of a bottle in litres or Fl Oz

Extrude a solid body around the bottle, with the top surface where you like to have the filling level.. Uncheck Merge result.
Select Insert/Features/Combine from the menu and select Subtract.
Subtract the bottle from the Extrution.
The result will be 2 solid bodies.
Open the Solid bodies folder in the Design tree and right click on the body which is outside the bottle and select "Hide solid body".
Select Tools/Mass properties from the menu. Tic off "Include Hidden Bodies/Components".
Click Recalculate and by Volume you can read the Bottles volume.

How can I calculate the volume of a bottle in litres or Fl Oz
once I have drawn a bottle shape.
Or can SW tell how big my shape must be for a certain amoun t of liquid.

Thanks

No.

This question might get more attention under a more appropriate topic than Sketching such as Parts

No that's not the only way, but its the fastest.

You could also select all the faces of the inside of the bottle and either knit them together as a surface or offset surface at a zero distance, then cap off the top with a Fill surface, choosing to knit and form a solid all in one go. Then you have mass properties.

You'll have to find a conversion for cubic inches to fluid ounces, which shouldn't be hard.

To change the volume, you could use a scale factor, or just change a dimension. and calculate how much it increases the volume, then figure out what the dimension needs to change to to get your target. There is no automatic tool for this.