>>over the winter i will be building a new cage for my tegu so i will be able to build my iguana a new cage by then. i have enough space for one i think. the cage would be 8feet long, four(maybe five)feet wide, and around 5 or six feet high. would that be ebough space for a single iggy?
It will be big enough for a number of years, but a single iguana needs an enclosure that is 1.5-2 x its total length in width...so a 6 ft iguana needs an enclosure that is at minimum 9 ft long. If your iguana maxes out a 5 feet, your 8 ft wide enclosure will be the smallest it should have. You'll find info on Iguana Age and Expected Size and Basic Cage Design Problems at my site. This information is also included in my Iguana Care, Feeding & Socialization (ICFS) article.
If the iguanas you had that you say died of old age were not 15 years of age or older, they didn't die of old age... You might want to read my ICFS article to make sure you've got everything you need, including access to the foods needed to construct a healthy diet for them. Tegus are a lot easier that green iguanas. 
>>also when he/she gets bigger they will be able to roam around in my room.
Good! That will enable you go to with a minimum enclosure size (still large at 8 ft for a 5ft ig, 9 ft for 6ft ig) and be healthier all around, especially when you set up basking and other areas for it in its free-roaming space. Think about using the top of the enclosure - you can set up a basking area up there, and since igs like to be high up off the ground, he will be able to get up higher (and feel more secure) on top of your 5 ft enclosure (6 ft tall is better) than he will be able to get when he is inside the enclosure. You could build a trap door into the top so he can get in and out on his own (when you open the trap door), as well as put a climber on the outside of the enclosure so he can climb from the floor to the top when he's free-roaming.
>>and to start with he/she will be in a 45 tall(3feet long 16"wide? and three feet tall.
Just keep in mind that, properly cared for, he will outgrow that in less than 12 months (assuming you get a month-old hatchling), so be sure to have his adult enclosure ready by then. Or build the 8ft enclosure first, and install your baby in there. So long as the UV light is close enough so that he gets the UV he needs, and the thermal gradients are where they need to be for him, and you put in a couple of hide boxes or visual screens he can hide behind, he'll be fine. There is no law that says hatchlings have to be in 50 gal tanks (just not any smaller than that!!!) to start - installing them right away into an adult-sized enclosure is just fine.
>>i have all of the heat and othjer lamps and the vitamins
Be sure to test out all of the equipment - using thermometers for the daytime and night time radiant heat sources and across the thermal gradient - before you bring your new iguana home. The time to find out that stuff isn't working is when you've got an iguana sitting there who is too cold to eat or digest properly. 
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Melissk
anapsid.org
Iguanas for Dummies