Not certain I follow you. HDMI connections are all the same size (other than some PC video cards that have a mini-HDMI port). Are you saying that the extender cable is still too large, as in too long, to fit in the space, or are you referring to the cable not fitting the connection on the TV?
If it's the latter, it sounds like you might be attempting to connect the HDMI cable to a USB port. They are near the same size, but a different shape and perform different functions. You can't connect an HDMI device/cable to a USB port.
Dan Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.
Could you send us the model number of your Roku device? It can be viewed by navigating to Settings > System > About or at the back of the player. Also, please make sure you are using zeros and not the letter O when you're inputting your serial number.