I would say no.
If you intended the spell to result in a single size large creature, that isn't what the spell does. It changes the caster's form and potentially size, it does not allow multiple creatures to meld together.
If you intended to get two size Large creatures, that also doesn't work. Assuming a normal size caster, the familiar will not be size Medium so it can't be polymorphed into a size Large creature.
I think you could use Polymorph Self to create two Diminutive creatures. Depending on the familiar, you could make it all the way up to two size Small creatures. The familiar would have to be size Tiny for this to work.