Hello from Asia!

I am a crocheter who is relatively new to knitting.

I already have knitting books and magazines with beautiful patterns -these inspired me to finally decide to learn how to knit.

I own only a few pairs of needles at the moment. I have already practiced purl & knit & some pattern stitches

Now I wish to try the other patterns in my magazines, but I discovered that there are so many needle lengths used in different patterns!

So now I do not know which lengths to buy that I can use for the most patterns.

For example: 1 pattern calls for a circular size 7 that is 80cm. Then another pattern calls for another circular size 7 that is 60cm. Do I need to buy them both? Or is it ok to buy just one - the longer (the 80cm) and then use it for the other project that require 60cm?

Also, what needle lengths are most useful to you for socks, for hats, and for something bigger, like sweaters & poncho? Or do you buy the length that is indicated in the pattern?

I wish to understand about needle lengths, please enlighten me.

I searched for the answer in my knitting book, but there is no topic about it there, so I am counting on your help so I can be guided on what to buy. As of the moment, I cannot afford to buy complete sets of varied lengths. So I wish to understand. I am sorry for the long post! Thank you very much for you help.