The basic rule of thumb we use for electrolytic capacitors is to use a voltage rating that is 1 1/2 times the voltage it may receive. The circuit boards are designed to physically hold a capacitor that will fit that size or slightly larger.
If you wish to learn this and other useful build tips, we suggest you RTFM. This means “Read the Fabulous Manuals” Barry posts on the forum. There is a wealth of useful information for all builders regardless of your experience level.