て and って sound different. The /t/ sound in the latter is longer (or you might perceive is as if the latter has two /t/ sounds).
This is called gemination. Gemination is rare in some languages (including English), so you might not be used to listening for it. One example is the /t/ sound in "hat trick" versus "Patrick". You might pronounce the t longer in the former.
In Japanese, all "big" kana and little っs should take up approximately the same amount of time when speaking (the rest of the "small" kana modify the previous kana, but don't change its length). It's as if each kana takes up one beat in a fairly stable rhythm. In って, you should be able to hear the rhythm resting on the /t/ sound for a whole beat.
おう doesn't have gemination, but its length should still be approximately twice the length of お. In the rhythm of one beat per kana, おう lasts two beats, お one.
I think I've heard that some Japanese teachers teach their students to practice this by clapping a steady rhythm while pronouncing.