Yes, if it's got cast/crew/movie info on the back it's a press poster. If not it's a speed. Either way they typically come in two sizes -- B4 (half a B2 vertically) or B3 (half or less of a B2 horizontally).
The B paper sizes are very precise, with each being half the size of the next higher number.
B4 would be half of a B3 poster (and about 10" x 14" in size), not half a B2 and is definitely not the size a vertical speed or press poster would be.
Horizontal B3 posters (which are 36.4 x 51.5cm or about 14" x 20") are most typically used in train cars (and called nakazuri posters). Up through the late 40s, vertical B3 posters were also a standard size for regular film posters (like the Japanese Casablanca Heritage sold in their last auction).
Pretty sure every speed poster I've seen has been that half of a B2 vertical size (about 14.3" x 28.6") Harry mentioned, but press posters can vary widely, from that size to smaller (as small as 10" x 20"). Can't think of a horizontal press poster that was as big as a B3 (though many are that length), but I have seen vertical ones that size.
It's important to remember that up until the 60s, many times the press sheet poster was the speed poster - as in there was no separate speed poster printed for the film like there might have been in the later 60s or 70s
