Here is some information to address some of your questions so far.
SIZING
For each BOO! pattern, we thoroughly test for mixed sizes. One of the reasons a lot of you sew is because your kids are not 'standard' sizing so we spend a lot of time during testing to try and make the patterns work for as many of you as possible with simple and straight forward adjustments to help you get a lovely fit.
Sew based on measurements! Using RTW (ready to wear / store bought) sizing will never give the best results. Sizing differs dramatically between brands and the fit of a t-shirt is a lot more forgiving than fitted garments like swimwear.
If you are going to the trouble of sewing garments and have the ability to take measurements, definitely take them, it only takes a few minutes!
DON'T WORRY ABOUT FIGURING OUT EXACTLY WHAT YOU NEED TO DO WITH THE MEASUREMENTS JUST YET, THIS IS ALL COVERED IN THE PATTERN AND DAY 1 OF THE SEW ALONG.
THREAD
For stretch sewing, use polyester thread in your needles (sewing machine, overlocker and coverstitch) for strength and durability.
If you decide you love stretch sewing, it is worth investing in stretch thread/woolly nylon. You can use stretch thread in the loopers of your overlocker and coverstitch machine or in the bobbin for your sewing machine (wind it on by holding the spool in your hand and using the automatic winder on a slow speed). By using stretch thread, you will get a little more stretch in the seam and it will be softer on the skin which can be important for those with skin sensitivities.
We stock Maxi-Lock brand of thread on the BOO! Website.
Maxi-Lock All Purpose Thread
Maxi-Lock Stretch Thread
Maxi-Lock Swirls (Variegated)
Polyester and Woolly Nylon can also be found at Spotlight/Lincraft or your local sewing store.
Maxi-Lock Stretch and Woolly Nylon are both Nylon threads. Woolly Nylon usually has more stretch than Maxi-Lock Stretch and is a little 'fuzzier'. Maxi-Lock stretch has a smoother feel and a little thicker so it 'fills' the seam better.
If sewing Cotton Lycra and woven fabrics, Maxi-Lock stretch is a popular choice because it can handle heat better than Woolly Nylon and you don't need to change your thread when moving between different types of fabrics.
Stretch thread can be used for rolled hems too (only needed in upper looper) and it creates a lovely full stitch. Maxi-lock is a popular choice for this because it is fuller and because of it's ability to hold up to heat better.
Heat test on high heat. Woolly Nylon melted and completely disintegrated but Polyester and Maxi-Lock Stretch were unaffected. 
NEEDLES
75/11 stretch needles are the best for sewing swimwear using your sewing machine. Chrome finish is great if you can find it because the needles are stronger and less prone to breakage.
If you want to try twin needles, you can use either 2.5 or 4.0 Stretch needles. I like 2.5 wide but it just comes down to personal preference, either will work for this pattern.
If you are lucky enough to own an overlocker and coverstitch, you can just use the needles you would normally use.
All of these needles are available on the BOO! website. Consult your machine manual to make sure you are purchasing the right needles for your needs.
ELASTIC
This pattern requires 10mm woven swimwear suitable elastic.
Avoid braided elastic, it is too flimsy and narrows when stretched making it difficult to use. Don't use non-roll elastic, it is too firm to sew through.
You can buy the 10mm elastic on the BOO! website in 25cm increments (cut continuously) or as a 100m roll.
That's all for now. See you back here for the start of the sew along very soon!
