How I know table size on the server

Size can mean different things in Oracle:
There is allocated space, data size, size also used by indexes, etc...

If I had to guess at what you are after, try this:
select bytes from user_segments where segment_name='SOME_TABLE_NAME';

I will post my table name ?

>>I will post my table name ?

I don't need your table name.  Just use it in the query I posted instead of SOME_TABLE_NAME.

depending on the data types you may need to look at related segments too

in particular  LOBs and XMLTYPE that are stored in their own segments.

you may also want to include the indexes of the table in order to get a fuller representation of how much space the data takes.

all of these can also be found in the USER_SEGMENTS, ALL_SEGMENTS, and DBA_SEGMENTS views.

I hope you have an oracle dba ...
He should be able to give you the fine details

Yes I have dba , but I would not to add big size on the server ,  however , let say if the size of tables in access is 1000 kb , is may be will be in oracle in approximately same average ?

well, your dba really needs to get informed about this
you'll get a lot of valuable input back

whenever someone comes to me for an item like that
> we look in what database (and tablespace and schema) the data should go
starting is usually in TEST, then QA then PROD
> we look at what size it will be
> we look at the grow rate
> we look at the number of users (and concurrent ones)
> we help upload the data (if asked, and recommend our assistance in case of massive amounts)
> we follow up for performance

your dba would also be able to tell you how this is all done in your company
and provide you with guidelines for apps, data, backups, etc, etc ...

thanx for all experts , i may be now more understanding about it
but stil one question ,
slightwv

There is "used" space, and "allocated" space.  They can be hugely different.

ok , this is clear now , but :
1- size of tablespace calculated with which one ?
2-also , when take backup , the size of backup calculated with which one ?
3 -  what is you advice to create a table with minimum size of space to save spaces in the database , in other word , how i can determine what is the suitable size of each field i want ,
4- and which fields types consumes more spaces than others ? varchars , numbers , dates ?

thanx for all advice's
i distributed the points as per what i got better understanding
and it doesn't necessary means that one answer is better than the other
thanx again for all