What was the percentage of land mass in prehistoric times when temperatures were high enough that we had no ice caps?
The real cause rising temperatures is not frozen ocean turning back into water but the volume of the ocean itself expanding (water expands just like everything else when you heat it), and ice on land melting and flowing into the ocean (adding more water). 

Ice sheets can be tricky to predict but it has been included in this report . Although for the most part ic just say whether it exists or not reconstructing total volume of ice is currently not something we can determine accurately.  

This chart from that report show how ocean temperature and sea level correlate. 


This paper shows how the two factors compare, they are are mostly comparable but ice melt (mostly glacier) is starting to overtake expansion. Great chart on page 1157.  

As for historical rise, earth used to have two ice sheets, one at each pole (this was responsible for the Asia-North America land bridge), but the northern one has almost completely melted, except for greenland, that is why we had historical sea level rise. There used to be thousands of feet of ice over boston. Land locked ice sheets are very sensitive to temprature, easy to melt, island ones like greenland and antarctica on the other hand create their own sort of insulation by altering circulation. These latter ones are much harder to melt, you need a much greater rise in temprature, which we have created. The last big melt turned england into an island.