Bulb size ? New one on me and I thought I knew something about Yanmar tachs.
Don't bother with the original Yanmar tach for a replacement. Get a VDO which is top of the line and has the same face hole as the old Yanmar tach. The VDO reads alternator output, ring gear teeth/rev, Hall Effect, heart pace makers, fit bit watches, etc, etc. You name it, the VDO tachometer will count it. Installation and calibration are extremely easy.