How do I know how many watts my electrical items use?
There should be a tag (or an equivalent stamped into the molding) on the appliance. It's typically on the back panel, or the underside, or for smaller handheld items like hair dryers on the side of the casing or the handle.
Sometimes the label won't specify the wattage directly, but will instead tell you the maximum current the appliance will draw (in amps). For basic lighting and heating devices (hair curlers etc.) and electronics like radios and TVs, you can get the wattage by multiplying that current by the voltage. So if it says 120V, 0.5A, it's 120 x 0.5 = 60 watts.
It gets a little more complex with things which include motors, since the product of the voltage and current can come to more than the actual wattage, which is why motors will typically give the current rating. So in pure technical terms, your sewing machine motor is 120V x 0.85A = 102VA (Volt-Amps). The actual wattage will be a little less, but it is the VA rating (the product of the voltage & current) you need to use when selecting a transformer anyway.
I was very careful not to bring any electric items. And one of the items I brought over was my keyboard and Korg is already sending me a new plug for it. But I do have speakers for it would they also work with the transformer
So long as the transformer is rated high enough to run them. Again, there will be a tag on the amplifier/speaker combination indicating the power consumption, although unless we're talking about powerful speakers to fill a dance hall, it's likely to be a few tens of watts at most.
Then the last thing I have is my blow drier, I don't need a transformer since it already has a button to change into 220, but it would be nice to use the other options on it again ( I can only use high atm).
So the 220V option bypasses the ability to switch to low heat? That sounds an odd arrangement, but then dual-voltage hair dryers are quite unusual anyway.
Since those are all the items i need to plug in and I really do doubt I would ever be playing my piano, sewing, and blow drying my hair at once
Well, you never know.......
I am thinking the one at the top of the list you posted would suffice. Would I be correct in my assumption?
The 500VA unit? What's the power rating of the 120V blow dryer? That's going to be the one which is the most power hungry, possibly well in excess of 500 watts.