Take a tape measure and size up the area where you intend to put your twin extra long sized bed so that you'll have a more exact idea of how long the bed should be, and bring this measurement with you when you go shopping. You should make sure that the bed you choose will still allow enough space to easily enter and exit your room and won't block access to windows or closets.
Take a look at the different types of twin extra long sized beds available by logging online to a variety of different retail websites. This should give you an idea of the range of these beds on the market and allow you to gauge the differences in pricing, materials and lengths offered by different stores.
Head to local retailers where twin extra long beds are sold, as you'll be able to see the beds in person and test them out, giving you an advantage over simply purchasing the bed online. Once you've spotted a few twin extra long beds you like, lay down on each bed and stay there for at least a few minutes to get a true feel for the bed.
Consider all of the options you've tested out and choose the twin extra long sized bed that fits your needs - including your bedroom size, your height and comfort requirements.