Can anyone tell me which is the best software for Designing a clothing company Tag?
Also any preferred/standard size (if any) or special requirements for designing a tag?
Standard tag sizes
The size of the tag will depend on the type of tag you want to make, see examples Clothing Tag Design Guide. You could create any size tag that you want (if you're printing yourself). It is best to contact a printer for sizes if you're printing the tags or getting them embroidered somewhere else.
One example of a standard tag from the above link is 1.25" W x 2.5" H. This is the tag dimensions and not the working art area (I couldn't find any definitive answers for a "standard" tag size).
Putting the bar code
The bar code will most likely go on a separate tag. These tags differ in that the clothing tag is for sizes and the price tag is something that you can detach from the dress.
What to put on the tag
This is an opinion question as you could put whatever you want. I have usually seen the companies logo and the other side is washing instructions.