MM, you need to give more information. For starters, what are you repotting? Where are you? Is this an annual? perennial? shrub? tomato?

I am repotting a 150 cm tall mango plant which is in a 18 L pot. The present pot is a nursery black pot which looks kind of small but it's twice as bigger as the pot which came in from the nursery about 3 weeks ago. So I repotted it in a slightly bigger one but looks like needs a bigger por. When it winds the actual pot seems not very stable. I am in Tirana, Albania and growing it in my balcony which views east-south.

Unfortunately That photo doesn't look too good. I will try to take another one later.

Even without seeing a photo, that sounds like a very small pot for such a tall plant. I have 2 suggestions. First, if you do a search in Gardenweb you will find several threads on Mangos. I can't offer any help as I know nothing about growing them, but you may find some advice there on the best size pot.

Second, I find that heavier pots (wooden half-barrels; ceramic pots with drainage holes) don't topple over, and if you can put in a support and attach it to the balcony wall or railing, that might help. That's what I do with my tomatoes.

Continue to pot up as large as you like. When it gets too large for you....consider using for bonsai material.☺