Your estimate of the wrench size sounds about right. What you may want to do is buy the replacement Oxygen sensor before you remove the old ... that way you know for sure. If you are dealing with original manfacturer sensors, get ready for a pleasant surprise ... they are not difficult to turn ... anti-sieze was used originally ... and you should use some on the new ones ... in fact, some replacement sensors come with the anti-sieze in-place.
You may want to make a 'snubby" wrench by grinding a standard combination wench in-half. These can be really handy. And you can get a special wrench, just for sensors, on the AutoZone tool loan program.
The real question is "Why are you replacing the sensors?".