Normally length of patch is radiating length lambda(g)/2 and width is related to edge impedance of patch. You can make a square patch *half of guided wavelength) and add an inset feed to match the impedance. lr use probe feed alternatively. Square patch can generate two degenerated modes if properly excited hence it is normally used in circular polarizations.

you can find all formula from net also use RF calculator .u will calculate exact length and width of patch otherwise u can use a simple patch formula manually also like lambda/2

Dear Mausa,

I am agreed with the answers/hints given by Dr Muhammad and Dr Anil Gupta.

Best wishes

Mr Mausa

You can make it square,  following the first post rule from Muhammad. The only thing is that the impedance right on the edge will be different from that of a rectangular one, but this you easily adjust by sticking the feed line deep into the patch (inset feed).

Square patch antenna design formula given in Antenna design by balanis book . Please refer it. 

1. Determine effective dielectric constant 'epseff'' for the given substrate using standard quasi-static formula (Wheeler's/Scneider's Relations) available in regular textbooks like the one by Balanis or "Microstrip Antenna Handbook" by Bahl et al.

2. Determine guide wavelength (lambdag)=free space wavelength (lambda0)/sqrt(epseff)

3. Find equivalent line extension (deltal)by Hammerstad's formula  - again available in the mentioned textbooks.

4. Choose length of each side equal to lambdag/2 - deltal.

5. You will get the relation for input resistance for inset feed from Bahl et al and then use it to choose the exact feed location to match with the 50 ohm feed.

Thanks everybody for your esteemed cooperation , many thanks