To make sure your top fits, choose fabric that is at least as stretchy as your original tank top. 

This method may help you quantify how stretchy fabric is.  Pin a 100 mm crosswise fold of fabric.  Stretch it and measure how much extra it will go.  An extra 25mm means 25% stretch, an extra 50mm -50% stretch, 100mm - 100% stretch.  You get the idea...