in theory the field number (typically of the order of 22 mm divided by the transverse magnification (x100) or more if you use an additional magnification changer. In practice often smaller, for several reasons: (i) the laser beam is difficult to focus in the very small back pupil of a high-NA objective with a small pupil such as a x100, therefore the beam is smaller and hence the effective field of view; (ii) in objective-type-TIRF the field of view is often unevenly lit unless the beam is azimuthaly spun (see, e.g., van 't Hoff et al. 2008 and references therein), thus reducing the effective field of view; (iii) depending on your camera, the chip size is clipping the field of view, so to precisely answer you have to know the diagonal of the chip of your EMCCD. A reasonable value seems to be of the order of 50-80 µm field-of-view for your objective. Hope that helps.