default window size vs scrollbar1
With iterm set to use the system window restore policy, when I launch I get a single new window placed as far to the top left as possible without obstructing any existing window. Normal.
In 3.3.4, and starting around 3.3, if iterm is set to hide the scrollbar, the window dimensions match those set in the settings for default size. However, if iterm is not set to hide the scrollbar, the window size gets reduced by the width of the scrollbar, rounded up to the next whole character width. The window width then grows by the width of the scrollbar if you change the scrollbar setting to hide the scrollbar.
So for example, with hide scrollbar unset, and default dimensions 80x25, I get a 77x25 window when launching. If I set hide scrollbar, the window changes to 79x25. (Not 80x25 due to rounding up of the width for fractional scrollbar vs character sizing.)
For the 77 character-wide window, iterm initially actually thinks it is 80 chars wide. On launch, it shows a label of the size: 80x25, and 2.x characters disappear under the scrollbar. However if you touch an edge and resize, the window jumps and the indicated width shows the actually displayed 77-char width.
Any new windows I create (CMD-N) after the initial one, take on the correct dimensions regardless of scrollbar setting.
My workaround is to immediately close the default window on launch, and then open a new one.
I'm using the font 'Meslo LG M for Powerline' (for oh-my-zsh), rather than a system-supplied font. However I've always used this font and it's only since ~3.3 that the window size issue started.