The size will depend on many factors. Species, conditions given and such.
I see you are from Australia and in your summer now. You live where the climate in most areas is conduscent to year round growing outside. So fortunate for an Adenium grower.
Right now with you in your summer should be a time when they are thriving.
Make sure it is getting full sun, although filtered sunlight at hottest time might be better. Not sure of temps. now for you? Watering before the soil dries back or as needed. Give a fertilizing with something similar in ratios to 9-3-6 or 24-8-16 at 1/2 the recommended rate once a week and plain water other times, until you start getting into your fall. Then reduce strength or stop with fertilizing. Because you have such a long growing season there, you can in springs/beginning of summer, top dress with slow release fertilizer (comes in different forms, i.e. 4mths, 6mths) 10-10-10.

Are these seedlings Obesum?

Rick

Yes they are obesum, it was given to me as a present yesterday, so I am very new to this and want to make sure my little gift thrives!

We are in mid summer here and it is usually between 40-45 degrees Celsius during the day and drops to mid thirties at night. Very little rain and about 14-15 hour of sunlight around this time of the year. Very, very hot!

While I have your help rick, winter is cold here as we live in a desert, it can get down to about -2degrees Celsius at night but is usually back up to 20 degrees during the day, should I be bringing my baby inside when it is winter? If so are normal flouros enough to light my plants?

Cheers Tori

Tori, They can take hot sun, but I know that acquaintances of mine in Australia do give them some filtered sunlight during the hottest time of day only.
Here when the daytime temperatures get down to 15 degrees Celsius I make sure that mine are up against the house or in the greenhouse. If they go to 10 Celsius then they are definitely in the greenhouse and I do not like them to be wet or moist at these temperatures. Then you can end up with rot.

There are alot of very helpful people here and I believe that Sam (may be wrong with the name) is from Autstralia and may come on and give some sound advice being from the same part of the world.

Do not be concerned about posting as many questions that you may have.
Rick