Set VM Configuration > Hardware > Video > Resolution to More Space. This will revert font scaling in Windows to 100% and you'll be able to change it to your liking. With 250% scaling you'll need to disable mouse pointer shadow, otherwise up-scaled pointer will not be h/w accelerated.
On the other hand, many Windows apps' UI don't handle such high percentage scaling properly. You may be better off setting VM Resolution to Scaled and 125% fonts in Windows.