Since my tab isn't in front of me I can't be definitive, but you can long press the weather widget and it may give you the option to resize (I use Nova Prime, and I know that it allows resizing of some widgets, but I am not sure about the default launcher).
If resize is not available, remove it, (long press the widget and drag to remove it) then add it again, using whatever size option (of the available sizes) that you prefer. To add, long press the empty space where you want the widget.