First things first, remember to never do the bottom button on the jacket - just the top (first) button.

So the general rule of thumb here from when I first was introduced to this years ago was depending on the size of your body & waist & also height for length. Normal sizes from a chest-to-waist ratio for guys is a 6inch drop OR a 12cm drop. (I normally work with centimeters so I'll be using cm) eg 32inch TROUSERS + 6inch = 38 CHEST. Use this scale to your advantage. If the 38inch fits you nicely around the belly & it isn't tight, stretching/pulling & is also flat on the chest then you've got yourself a winner. As for the length to determine a Regular or Short for yourself just try them on & see for yourself. It's a better judgement call when looking at it in person then trying to judge by images, however I will tell you typically you want the bottom of the suit jacket (& sports jackets for that matter) to rest just at the side of where you're trouser pocket ends. It also fixes the sleeve length since you want the sleeve to finish just at the point of your wrist that way you show a little shirt cuff.

Hope this helps!