Solution

The total magnification of a microscope is the product of the magnification at the objective $L/f_1$, where $L$ is the distance between the objective and the eyepiece, and the magnification at the eyepiece ($25$ cm)$/f_2$, where $25$ cm is the near-point distance of human eyes. Thus, if $L$ is increased by $l$, the total magnification will increase by $25l/(f_1f_2)$ cm.

If the microscope is enlarged to $k$ times its original size, each of $L$, $f_1$, $f_2$ will be $k$ times its original value (as the radius of curvature of each lens also increases). Consequently, the magnification of the microscope will be $1/k$ times its original value.