If I realise and show a VTE widget before calling vte_terminal_fork_command, the values passed to vte_terminal_set_size get ignored and the size gets forced to 80x24 characters. But if it isn't anchored before forking I can't set WINDOWID correctly for the child.

So if I want to set WINDOWID I have to realize the top-level window before the terminal widget is at a correct size. I can force a resize after the fork, but that causes incorrect window placement. In these circumstances trying to get the window to the right size before it's opened tends to result in spaghetti code.