The optical design of an integral field unit (IFU) based on image-slicers represents an interesting design exercise because of the small size of the components and the critical parameters to reduce optical aberrations. When the IFU to be designed is also intended to substitute an IFU prototype that is already integrated into a spectropolarimeter, it becomes even more challenging. The main reason to upgrade the IFU prototype is to cover a larger field of view (FOV). One of the work packages of the European project "Getting Ready for the European Solar Telescope" (GREST) had as its objective to find the best option to design such an IFU. After analyzing the requirements and the design considerations, the optical design pointed toward an IFU that doubles the FOV, reaching a value of 40.5 arcsec2. The designed IFU is composed of two image-slicers coupled together, each one with eight mirrors of 1.8 mm × 100 μm that feed two slits to the spectrograph.