I have a 7 speed Sondors Fold EX bike that I want to change the chain ring to get more resistance while pedaling in higher gears. I am finding that I cannot get past level 2 PAS without "running out of pedal" (I think is the term). I do not want to make changes in the rear cassette at this time. I'm presently running a 48 tooth gear on the bike. How much bigger of a gear should I use to give myself more resistance when pedaling? Do I have to get a longer chain also? Also, do I need any specific type of gear? Can I get one on Amazon? Thank you!