Thumbnail image: Recommended size 1200×627 (Aspect ratio 1.91:1) - jpg, png etc...
Brand logo: 1:1 ratio
Long headline: 50 character max (use long description)
Short headline: 25 character max - required field, though it won't display on Sharethrough inventory
Long body text: 140 character description (use long body text)
Short body text: 90 character description - required field, though it won't display on Sharethrough inventory
Call to action : Not supported by Sharethrough.
Note that you'll need to populate the CTA field as DBM requires it in order for the ad to run, but the actual placement will not have it upon render.
Additional information may be found on the DBM site here.