We　fabricate　850-nm　vertical-cavity　surface-　emitting　lasers　(VCSELs)　with　oxide-aperture　diameters　of　5,　7,　9　and　11　mu　m　and　study　the　effect　of　oxide-aperture　size　on　the　threshold　current,　output　power,　differential　quantum　efficiency　and　wall-plug　efficiency　of　the　devices.　We　characterize　the　small-signal　modulation　bandwidth　(f(3dB))　of　the　devices　and　explore　the　impact　of　oxide-aperture　size　and　bias　current　on　the　modulation　bandwidth.　We　analyze　the　relationship　between　electrical　power　and　the　f(3dB)　of　the　VCSELs　with　different　aperture　diameters.　The　fabricated　850-nm　VCSEL　with　the　oxide-aperture　diameter　of　7　mu　m　at　room　temperature　shows　a　maximum　f　3　dB　of　nearly　20　GHz　and　an　output　power　of　9.09　mW.　When　the　oxide-aperture　increases　to　11　mu　m,　the　f(3dB)　remains　above　17　GHz　and　the　optical　output　power　exceeds　14　mW.