Propagation and mode coupling within relatively short ( $\sim 1–10\mathrm{m}$ ) large core, nominally multimode, fibers are of interest in a number of applications. In this research, we have studied the output beam quality and mode coupling in various fibers with core diameters of $100–400\mathrm{\mu m}$ and lengths of $2\mathrm{m}$ . Output beam quality $(M^2)$ and mode-coupling coefficients $(D)$ have been studied for different clad dimensions, numerical apertures, and wavelengths. The mode-coupling coefficients have been determined based on modal power diffusion considerations. The results show that D scales approximately as the inverse square of the clad dimension and inverse square root of the wavelength. Output from a $2\mathrm{m}$ length fiber of $100\mathrm{\mu m}$ core and $660\mathrm{\mu m}$ clad fiber is close to single mode ( $M^2=1.6$ ), while output from a $200\mathrm{\mu m}$ core and $745\mathrm{\mu m}$ clad fiber also has high beam quality.