HELP! What gauge/size are these 3 wires?

I am in the process of swapping the internals on a regulated vaporizer (ecig/rx200s) but I'm not sure what size the wires are that I want to replace. 1.) Can anyone tell me the gauge/sizes by these pictures? I'm thinking the big ones (black and red on the board) are 14g maybe? And the medium (short red)... show more I am in the process of swapping the internals on a regulated vaporizer (ecig/rx200s) but I'm not sure what size the wires are that I want to replace.

1.) Can anyone tell me the gauge/sizes by these pictures? I'm thinking the big ones (black and red on the board) are 14g maybe? And the medium (short red) is 16g? And smallest 18g? but I want to be completely sure so any help would be so great.

2.) Where would be a good place to buy wire like this? eBay is the only place I've been able to find what I need but the prices aren't the best.

3.) Does it matter if the wire is "pre-tinned" when it comes to soldering?
And I'm assuming "lead free" solder is the most conductive option? Or does it not matter?

And last but not least;
4.) Is it a good idea to upgrade the stock wire gauges by just one size bigger? Or are they that size for a reason and should just get direct replacements of the same sizes as stock?

I apologize for the ignorance in these questions but I'm honestly new to soldering and just want to make sure I do everything right as I'll be using this device everyday.

Thanks a million!
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