I saw a full scale prototype/model in the Stirling lab. I think a strong person could have lifted it up. It’s just several lbs of uranium with several heat pipes coming out of it connected to Stirling engines. I don’t think they have enough actual Stirling engines because the ASRG project to make them was cancelled so I think they use simulators for the heat load.
Hard to tell what the weight is, of course. Judging from the size of other objects in the picture (the lift point and the connectors) it's very roughly around four cubic feet. Which is actually pretty awesome; the equivalent solar panels would be significantly larger.