Use a big button size.
The use of big button size in applications for older adults has two purposes, the first purpose serves to help people with vision impairments in noticing the button and the second purpose serves to help people with motor problems in pressing button accurately.
Some button sizes are recommended, for example a button for a finger selection is recommended at least 20 mm square, meanwhile buttons with 20mm by 31.7 mm are more appealing to users. Minimum recommended button size is 10mm, for older adults 11.43 mm, a slightly larger. Relatively the space of the buttons is recommended a space between 3.17 mm and 12.7 mm to lower performance error rates for older adults.
Using the Android Tablet to Develop a Game Platform for Older Adults, 2011
Touch Screens for the Older User,2011
Design Principles to Accommodate Older Adults,2012
Touch Screen User Interfaces for Older Adults: ButtonSize and Spacing,2007
Accessibility, Buttons, Content, Elderly, Large-Elements, Motor, Vision , Target Design.