My wife does, indeed, now have a "Horsey" jacket.
Sorry Brian, the smallest is the Polo DB POW and its a 40R. I'll let you know if it measures smaller.
There are varying sizes. I've added the tagged sizes below, but I haven't gotten around to measuring yet.
L to R: USA made JAB shawl collar tux jacket 45R, Brooks Brothers peak lapel, one button tux, two Ralph Lauren Black Label peak lapel two button suits 42R, two Corneleani made RL Polo suits (black 100% flax two button 42R, 6-4 double breasted POW plaid with a sick purple overcheck 40R), RL Polo 3 button herringbone SC 46R (again Corneliani), two 100% cashmere 3/2 sack SCs 42R for the trad crowd as well as two Polo Made in England Fair Isle sweaters (one shawl collared) XL, also for the trads.
Not pictured: Alden for BB #8 shell cordovan penny loafers 10.5D, Alden pebble grain PTBs 10 C, Nettleton? shell cordovan PTBs 10C, Loro Piana vest and 1992 Olympic team jacket for the wife, Five RL 100% silk knit ties including a Purple Label (although the four blue labels are made exactly the same). Two D.O.M. ties. Two beautiful vintage BB madras ties. A vintage 3/2 sack narrow lapel madras SC (which my son immediately claimed). Two pair of Zegna jeans (mint) 33, a pair of RL Purple Label slacks, two pair of RL Polo (Corneliani) slacks 38 (one NWT, but has a repairable hole NA) and a mint Cat Studio (ask your wife) Ski Colorado pillow for $5