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I know that birds evolved from dinosaurs (or a related clade). However, some dinosaurs are large and others are small. Did birds evolve only from a line of dinosaurs that stayed small throughout evolutionary history, or did the ancestors of bids get large at some point and then shrink?
Concretely, what’s the largest known animal among the ancestors of birds?
This is actually a very difficult question to answer - because how far back do you go. The clade Avialae which includes the true birds is fairly recent and contains a lot of new dinosaurs(! -debate over feathers), but the clade Avialae is included in the theropod (Theropoda) group of dinosaurs, which includes such things as Tyrannosaurus rex, Spinosaurus, and Giganotosaurus, all of which are fairly large. However, these are almost certainly sister clades to that of the birds rather than actual ancestors.
However, Avialae is a made up clade with no actual fossil representing it; it is purely hypothetical as an ancestral marker for birds.
It seems that there is quite a bit of debate among paleo ornithologists as to which groups of dinosaurs are most closely related to birds. It is thought that things like Archaeopteryx are part of the bird ancestral clade, but even this much is uncertain, as there are a number of other feathered dinosaurs within the larger clade, that definitely did not evolve into birds. You might consider Neotheropoda a suitable cut-off, as this contains the true birds, but is still quite diverse. You could also cut off at Averostra, which includes Aves, and Ceratosauria, a group of quite large feathered dinosaurs.
Having said all that, we are very unlikely to even know which species are directly ancestors of birds, it seems that there are a lot of species that are very similar and all closely related, but making the choice of which is down to experts and a paucity of specimens. See this bit on Birds for some of the difficulty in defining them. If I had to make a choice, I would probably go with Ichthyornis as a definite ancestor, but I am no expert (I'm a virologist...).
The largest bird was Vorombe titan, 3m tall and weighing 650kg. It is likely that this was larger than any of its ancestors.
Working back through the lineage of birds in Theropoda, the animals that are on the avian linage are small. One finds some early therapods that may be close to the common ancestor that were "largish" but smaller than V. titan. Going back to basal Archosauriformes one finds a side-genus like Proterosuchus, a crocodile-like animal that could be 3m long, but not as massive as V. titan. It is possible that there is a large semi-aquatic creature in the archosaur lineage, but it is more likely that the common ancestors of crocodiles and birds were smaller. The megafauna of the Permian were all synapsids and more closely related to modern mammals than to birds.
Prior to that, there are lizard shaped creatures, salamander-like creatures, back to lobe-finned fish and so on. No megafauna.
None of these potential ancestors is as large as V. titan. I'd conclude that the largest animal on the "bird" lineage is this type of Elephant Bird.
Perhaps this is not so surprising. Large creatures are vulnerable to extinction. The common ancestors of the extant clades tend to small adaptable generalists.
