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The mechanical performance of a composite is greatly related to the load transfer capability of the interface between the matrix and the reinforcing fibers, i.e. the fiber/matrix adhesion, which is enhanced by a surface treatment called sizing. The original sizing of reinforcing fibers is removed during recycling process, which is recognized to contribute in typical issues of recycled fibers, namely uneven fiber properties and poor fiber/matrix adhesion. Applying a new sizing, a process denoted here as resizing, can help mitigate the issues. Furthermore, the sizing has a major role in improving the processability of the fibers as it contributes to the distribution of the fibers in the matrix. Proper distribution, along with the fiber fraction, are highly important for the composite performance. These properties are ensured by proper compounding. Here we demonstrate and validate the process steps to resize and compound recycled glass and carbon fibers with thermoplastic matrices. We found that at a relatively high sizing concentration, the compounding of all tested material combinations was possible. The resizing of the recycled fibers improved the compatibility at the fiber/matrix interface. It was concluded that recycled fibers can be used to replace virgin fibers in automotive industry to allow weight reductions and to promote circularity.