Described are ferromagnetic resonance (FMR) studies of the long wavelength magnetostatic wave modes of a single crystal yttrium iron garnet (YIG) slab magnetized by an in-plane field. A resonant circuit comprising a coil, 50 m in diameter, is used to excite and detect the YIG microwave absorption spectrum. By changing its orientation and position over the sample surface it is possible to excite selectively different series of magnetostatic modes. The observed resonance fields are in good agreement with those calculated numerically from the Damon-Eshbach dispersion relation. The effect of heat currents on the magnetization dynamics in insulating magnets is explored. For a YIG crystal 10 mm in length, 2 mm in width and 25 micrometers in thickness, a temperature gradient of about 20 K/cm is sufficient to suppress the high-order longitudinal magnetostatic modes.