Averrhoa carambola has a tree-like growth form and can grow quite tall in tropical regions. In
Propagation of Starfruit trees is similar to most woody tropical plants, in that they can be effectively propagated by seeds and cloned by cuttings. In the case of growing Starfruit trees, growing by cutting or by air-layering would be ideal. I have no experience in propagating Starfruit trees this way because I don't have access to any live starfruit trees. If you live in a tropical region and have a nearby nursery or friend that owns a starfruit tree then your best option may be to propagate vegetatively through cuttings. For all others propagation by seed may be your only option. Starfruits can be purchased in specialty grocers and a single fruit can yield a dozen seeds. If you are unable to find a Starfruit you could try buying seeds on the internet. It is important to get seeds that are fresh as these will have the highest rate of germination; therefore plant your seeds as soon as you get them.
Growing Starfruit trees by seed: I have experience growing starfruit seeds from the internet and from fresh fruits. The key is to keep the seeds moist and warm. This was easy for me in my greenhouse as it is constantly moist and warm, I simply sowed the seeds about 1/4 inch in soil. Alternatively you could place the seeds in a moist paper towel in a plastic bag and place them in a warm dark place in your house (On top of the refrigerator). In optimal conditions the seeds may germinate in a week but I find that germination is typically closer to 3-4 weeks.
Growing Starfruit trees is tricky, even in tropical greenhouses. Seedlings often grow very tall with minimal branching making them prone to falling over or severing the stem. This may be caused by too little sunlight (they grow best in full sun) or by early application of fertilizers. You should wait until your starfruit is a few years old before you fertilize it. If you find that your Starfruit tree is getting too tall and is not branching you should cut the top off (The small growing bud at the top of the plant = apical mersitem); by topping the plant you force the plant to grow axillary branches and assume a bushy growth form. Starfruit trees grow best in slightly acidic soil that drains well. Overwatering can be a problem for Starfruit trees and you should not water the soil if it is already very damp.