Dinosaurs and insectivores
Colbert (1955) Evolution of the Vertebrates
Were the first placentals (mammals other than marsupials and monotremes) tiny animals living in the shadow of the dinosaurs? The idea predates cladistics and molecular phylogenetics (see above). Most fossils from the Mesozoic are indeed small.
A recent study (here) took another approach based on the fact that there is a faster divergence in GC3 in long-lived species than in short-lived ones (GC3 is the percentage of guanine and cytosine at the third position of gene codons). Body size correlates to longevity.
The authors constructed a tree based on the genomes of 33 placentals with two marsupials and a monotreme as outgroups. They then estimated the ancestral GC3 at each node of the tree including the most recent common ancestor of placentals. From this last value they could estimate the longevity and body mass of early placentals. The surprising result was they had a life span above 25 years and a body mass above
1 kg (less if arboreal).
Looking at mammals as a whole (not just eutherians/placentals and metatherians/marsupials), the fossil record offers several examples of medium to large mammals such as Sinocondon from the Jurassic and Repenomamus from the Early Cretaceous (elegantly reviewed by Luo here). It is not beyond the bounds of possibility that the most recent common ancestor of placentals was in a similar size range.