You can set the minimumSize property on the defaultRowStyle object of your grid like so:
grid.defaultRowStyle.minimumSize = [NSNumber numberWithFloat:grid.frame.size.height/2];
Or you could implement the delegate method shinobiGrid:styleForRowAtIndex:inSection: to return an appropriate SGridColRowStyle object, letting you size each row dynamically. (The first option will be best for your use case, I just included the latter option for completeness.)
Set the defaultRowHeight on your grid as shown below (defaultHeaderRowHeight for header cells):
grid.defaultRowHeight = [NSNumber numberWithFloat:grid.frame.size.height/2];
If you wanted to size the rows dynamically (again, probably not what you want in this situation) then you could implement the SDataGridDelegate shinobiDataGrid:heightForRowInSection: method, returning the correct height you want for that row.