Eileen, It is difficult to "quantify" the effect on resale value without a better definition of "no backyard". Is the back property all hardscaped and filled with plantings? Do other homes back up to the back of the home, providing little or no privacy? Or is there privacy provided through privacy fencing, or privacy landscaping?
Usually, when doing a market analysis of a home's value, lot size does not have a huge impact on market value unless there is acreage tied to the property. Even then, some buyers don't care about the additional land, so they are not willing to pay more for it - for example, an empty nester who would not want the maintenance of a large backyard. What are the neighboring homes like? Do they have similar back yards? Generally speaking, the lot size, elevation, etc. may make have an impact on how quickly the home sells, but not so much on market value. If I knew more about the specific property, I could provide you better information. Feel free to contact me, if I can help your further with real estate needs in the Des Moines and surrounding IA market. Thanks!