There are no restrictions on the size of a schematic symbol, but you should match the size to your other symbols to achieve a consistent schematic design. As PPL (PCB Parts Library) is the main component resource provided this should be used as the default standard.
The default technology file uses a 50 thou grid.
Some standard symbols from PPL are shown below with their respected length and width.
The styles used in creating the component schematic symbol are (viewable in the design technology):
1. Line style 'pin' which has a width of 6.
2. Line style 'thin' which has a width of 10.
3. Text style 'default' which has a width of 50 and a line width of 5 using the "System Stroke Font".
Using these details as a guide you will be able to create consistent schematic symbols.