The rise of mobile application is inevitable. Every year, the number of mobile application is increased. It is important for mobile application project owners to calculate the required resources before building a mobile application. In software metric, Use Case Points method is able to count software size of mobile application based on their functionality. This method utilizes use case diagram as their computation factors in the estimation process. Moreover, two other complexity factors are also considered in this method, which are: Technical Complexity Factor and Environment Factor. In this paper, we present software size calculation of QuestDone Mobile Application using 7 steps use case points method. QuestDone has been implemented, but we do not know its software size (i.e. how big the software, how much it cost, how many people is needed). As the result from use case points method, the Use Case Points value of QuestDone is 126.88 with Effort Estimation equal to 889 hours. The software size estimation process of QuestDone Mobile Application detailed in this paper can give an insight to project owners to count software size of other similar projects.