1st Step: Go to the form where datagridview is added
2nd step: click on the datagridview at the top right side there will be displayed a small button of like play icon or arrow to edit the datagridview.
3rd step: click on that button and select edit columns now click the attributes you want to increase font size.
4th step: on the right side of the property menu the first on the list column named defaultcellstyle click on its property a new window will open to change the font and font size.