The tool for planning on the website is not reliable. Count how many days you might need and start from there.
You'll probably need a Global Pass. Keep in mind that high-speed trains in France need a reservation. Cross-border trains are quite popular so book well in advance (17€). You can avoid them by taking a regional train across the border too.
Depending on the route, if you only will travel to/from one city in Germany to one city in France, and when you will be travelling, it might be cheaper to buy normal tickets with DB's Sparpreis Europa.