In three-dimensional (3-D) through-wall imaging (TWI) applications, imaging per- formance is directly affected by the configuration of the arrays. In this paper, the concept of equivalent array is introduced to design the 2-D MIMO array. A method named sub-array seg- mentation is used to circumvent the de-convolution process between the equivalent array and MIMO array. To get the optimum array among the results after the de-convolution, a concept of comprehensive size is used to evaluate the physical size of the MIMO array by using the informa- tion of sub-array segmentation, which is inspired by the split transmit virtual aperture (STVA) that has the shortest physical size given the virtual aperture. Finally, a 2-D UWB-MIMO array is designed for TWI as an example to validate the proposed method.