The first question you need to answer is will this be a house built to sell as an investment or a home to live in for a long time? First, never get attached to an investment. You will overspend regardless, so don't add to it with emotion. Whether you choose a spec or investment home, plan where your overages will be spent ahead of time. Write down your plans. This will allow for a subjective review of your ideas from others. Take your time to decide now. Once you start building, there are payments and even the costs of remodeling of a new house that make changing your mind costly.
Will you be building in town or in the county? There are limitations to building in town on a subdivided lot. When building out of town, though, you take on more liability because there are no inspections. If you dream of acreage, then your lot size will not limit the size of house you can build. Choosing a builder and contractors you trust become more important than the footprint of your home. In contrast, in a subdivision, fitting a custom home on a particular lot can be nearly impossible if mismatched. To avoid this, just keep in mind you are choosing a subdivision/house plan combo. For example, if you must absolutely live in a particular subdivision, the lot may not be big enough to include a wrap-around porch or other similar amenities. Look at the homes currently being built to guide you as to what combination is a good fit when building in a subdivision.
Gather ideas of what you like and don't like. This is the best way to get exactly what you want in a custom or pre-sold home. The pre-sold home is sold before the construction ever begins, which reduces some of the variables for the builder and can save you money. In most cases, it is best to ask an architect to draw your home plan. Pictures and example plans are the best way to communicate what it is you actually want. Purchase home plan catalogs and magazines, and browse through them for ideas. Or carry a camera everywhere you go and spend your weekend exploring different home ideas.