First of all, creating a "coming soon" page might not be a good idea at all. We have a discussion about whether or not they are helpful which I hope you read and consider the arguments.
If you still think that you should create a "coming soon" page, then the best practice would not be to design for one particular set of dimensions, but rather to design responsively, meaning it should look good on all screen sizes.
If you're new to responsive design, I wrote an intro to responsive web design that describes how to design responsively in more detail than I can go into in this answer. If you have questions or comments I'd love to hear them. You might also be able to learn some things by reading through questions that have the responsive-design tag.
With a "coming soon" page, designing responsively should be quite simple because there is likely not much content at all.