With your pressure washer rating, you would be best off purchasing a 5/16" diameter hose versus a 3/8" size hose. When using an extension hose, you will lose 1 psi per linear foot that you extend the hose past its original length. In this case, if the pressure washer came with a 25' hose and you purchase a 50' hose, you will lose 25 psi of which would not be noticeable to you.
You can extend the high pressure hose as far as you want, just keep in mind the amount of pressure that will be lost as I stated above. However with your psi rating, the maximum length that I would recommend would be 100'.
I am posting a link below to a 5/16" hose that I would recommend for your pressure washer:
MTM Hydro 5/16" Hose