Nvarchar or varchar what is better use multiply of 2 or rounded full numbers?

Doesn't make any difference. Use the size appropiate for your data.

For instance SQL Server, if you look at the Anatomy of a Record you'll see that your size translates into record offsets that are dependent on the previous record in the table, null values and other factors, specially with row compression and page compression taken into account. By the time the field is accessed, any resemblance with the original declare size relation, vis-a-vis powers of 2 or powers of 10, is long gone. Also various elements higher on a query execution stack like join operators or sort operators or whatever, also would no benefit from powers of 2 sizes (I have no 'proof' linkes, but is OK if you take my word for it...). Neither does the TDS protocol when marshaling data back to client. And I see little benefit in the client too.

There's no reason to use multiples of 2. Set the field to match the estimated size of your data.

One number that is worth mentioning though is 255. Some database systems have a maximum varchar type of 255, though this is becoming rarer. I'm thinking mainly here of what are now very old versions of MySql. And so sometimes developers will set the column size at 255 or lower to ensure more portability.