From a design and value perspective, when you have only one bathroom - the rule of thumb is the bigger and better is best. However, as you have one bath, something to consider. Showers are still the most popular bathing method and if your current tub is a tub/shower, you need to keep the shower - so the size of the soaker tub from the standing in and showering point of view is very important. At the same time, people with small children still need to bath them in a regular bath. Consider that with a soaker as a parent would have to be attending the children in the bath. Can you fill low enough for children and lean over to attend them?
From a strict value perspective, keeping the above in mind, it won't add a lot of dollars to your value, but it may help the appeal of a one-bath house. If possible, the real value would be to add another (master bath) to the home. That will add as much as $10,000 (on average) to your home's value.