Yes, blue moissanites are a little harder to get a hold of since C&C stopped making them regularly not that long ago (they still come up second-hand now and then). They had a limited run of them not too long ago, but it was definitely a short production run and they did not last long. I am more of a solitaire girl, but I love blue stones (I have sapphire) including blue moissanite, so if you love the blue, and have seen it in person and like it, go for it! You will have a hard time finding them again, if you want it.
For a halo, I usually prefer the look of a “smaller stone” – like in the .50-1.25 carat range. I think a .75 would be perfect! However, this is just my own style and taste. Some people like large stones in halo and rock it. I just tend to like a more classic, simple look. With a halo, I find that the coverage goes up quite a bit, depending of course on size of sidestones. You could also go for a double halo, increasing your coverage even more!
Solitaires I like a larger stone, usually between 1.5-2ish for me on my hand (if we are talking rounds, if you are going with a cushion or something since they are deeper the carat weight may go up a bit). My sapphire faces up like a very good cut 2 carat round diamond (it is about 2.4 carats). Much bigger and I just do not like the height off my finger, the “look”, or practicality of it/risk of it banging or catching (as is I don’t wear my rings for the gym or running or anything).
At the end of the day, it really is all about your personal style and preference though. Some people like larger, blingier halo’s, and some people prefer simpler, smaller solitaires. Depends a lot on lifestyle, how much jewelery you wear, your overall style, your own hand/fingers, and so on!
I would recommend you go to a jeweler and try on some halos. You may either love them, or decide you don’t really like them at all on your hand.
Note: if you go with blue moissanite, just make sure if you are ever having work done on your ring you go to a jeweler well versed in working with moissanite, especially blue! The reason they were discontinued is because there can be a problem with them losing the blue colour when exposed to certain temperatures.