Can The Main Window Be Enlarged?

Yes, you can have it stretched to your liking since you are given the source file. You should know that although this is possible, you will need an experienced Flash developer to do this as Flash is a little touchy

Can You Add Flash Menu's

You can since the source is included, but again...you will need an experienced Flash developer

Sales Tax Calculated

As of now the sales tax is manually calculated, so you must preset the amount, it does not calculate automatically.