Why is Dragon Quest 7 PS1 considered one of the longest JRPGs ever made?

Please don't be rude

Because ita long a sturdy, like timber.

For someone who's only played the DS version of DQ5 (enjoyed it), how does 7 hold up? PS1 or 3DS? I actually could use a new JRPG fix.

I'd seriously recommend you to play IV or VIII first, and then, only if you wish those games were 3x longer, play VII.