How do you choose soldering iron tip size and solder wire diameter?

HKPhysicist said:

but my solder joints seem too fat, not as slim as theirs shown here

Show detailed photos. It's easier to advise when we know what your work looks like.

I did another board this morning.  How do you rate my job?

I applied much rosin which seems helping electrons on different metals to "see" each other.

When I was a boy, I used rosin to clean my iron tip which turned black even quicker!  

I did another board this morning.  How do you rate my job?

I applied much rosin which seems helping electrons on different metals to "see" each other.

When I was a boy, I used rosin to clean my iron tip which turned black even quicker!  

Those flux condensed between the joints` gaps!  Are there any tool to remove it?  Cotton stick cannot remove them.

I see.

Let me try with my worn out Colgate.

thank you for these honest photos. I have done soldering just like that.  From that experience, I think you need to heat the pins and pads before applying the solder.  In addition to deciding solder size and tip size.

some of those connections look very good!
Its a delicate skill to get the solder around the whole pad, without adding too much solder.

the pictures are a bit blurry, but it appears you have dimpling, which is where the solder has flowed, and sunk after heat was removed.
this happens when the parts, metal pads, pins, get too hot for too long.

as for clean up, a toothbrush, and anhydrous ethyl alcohol work well.

Sitting on top looking like a ball is usually a bit cold..not enough wetting. Apply the tip to pin and pad count to 2-3 then apply solder should wet well all around with nice radius. That flux is old school to much residue. Do not use plumber flux. I use 559 flux in a syringe or others in a metal can on ali xpress. To clean up 90%+ isopropyl alcohol in a AMAOE M74 bottle -I luv that bottle- just push down with qtip and its ready to go. Mechanic makes very nice sponges for SMT no hair like qtip model NC800 you can trim/cut them to size you need..have fun