The torso heart would still be almost the same size as a human heart's. This is because the blood volume that needs to be circulated in the unit time remains the same, so the work to do remains essentially identical.

Perhaps the reduced differential pressure in the aortic trait might lead to the left half being *slightly* smaller, but I wouldn't expect much. And you'd get significant complications in keeping the hearts synchronised to avoid straining either the hearts or the intervening vessels: a synced systolic stroke between torso and lower limbs would raise the pressure in the aortic trait significantly.

If you had two separate hearts, one each managing pulmonary circulation and global distribution, then those could be a bit more than half a normal heart's volume (they would be essentially the left and right halves of a normal heart, connected by a large valved artery instead of being back-to-back). But I suspect this would lead to a loss of hydraulic load in the middle, leading to a need for both hearts to be larger.

You can maybe decrease heart size by having contractile vessels (i.e. supplementing with peristaltic pumping). This too requires some synchronisation to work at speed.