bbPress.org

change the numbers to whatever you want

#bbpress-forums div.bbp-reply-content img, #bbpress-forums div.bbp-topic-content img {
max-width: 500px;
max-height:250px;
height: auto;
}

you add this into anywhere where you can have custom css

in bbpress.css in your child theme(you copy the original in your child theme)
in your child themes css stylesheet
the jetpack plugin module custom css
a standalone custom css plugin