Advances in vehicle technology open up both opportunities and challenges for human-machine interface (HMI) design in intelligent vehicles. Design guidelines for icons in central consoles of vehicles have been discussed in human-computer interaction community, but more study and innovation are needed in icon design. In this paper, we investigated the displayed line thickness and size of icons in the intelligent vehicle’s central control screen, based on international design guidelines and standards. The experiment involved 17 participants performing simulated in-vehicle secondary task. The result from the experiments shows that the usability of icons increases and the driver’s workload decreases as the icons get larger. We also found a set of recommended values for size and line thickness of icons in this context. Future work will focus on providing design guidelines for other aspects of HMI design in intelligent vehicles.