In terms of median size? I’d guess (with huge margins of error) about 50 kg and 500 g respectively.
But pterosaurs don’t work as well as a design in very cold or very densely forested environments, so there were open niches. And dense forests are the current best guess for the types of habitats that very early birds inhabited.
What about that is not reasonable?
Elongated scales that can be manipulated for display become lighter and more elongate, allowing for easier manipulation and more impressive displays; they continue shifting in form towards something vaguely feather-like. This extra surface area proves to promote a weak ability to glide in jumping between tree branches. Improved gliding ability is then selected for to increase ability to go between trees, and this continues to increase until partially-powered flight is possible, and eventually, fully-powered flight.