You can set the connection pool size through Admin 
Console...i.e. Connection Pool -> General -> 
Configuration...there you will find all type of settings, 
its cache size, no. of connection pool and much more.

Note: User Advance option for more features.

Yeah .pranab is correct ..

hey pranab , wot u guys are talking abt . i think u guys 
didn't get the question.

hey vamsy dear, pranab is fool , don't belive on him .

send me correct answer otherwise don't open this

yes , pranab is almost right..you can find from admin 
server then you need to go into services there you will get 
connection pool where you can reset the size..

Yes.we can set connection pool size in weblogic server 
through admin console.At first you have to login to the 
adminserver and after putting username and password you 
will see connectionpool within service configuration,then 
you have to go through the connectionpool and there will be 
two tab one is configuration and another will be 
monitoring.From monitoring tab you can view maxcapasity and 
highconnection of connectionpool(jdbc).From that you can 
customize the value .

you can also check it  by clicking testpool.
Expected result:-connection sucessfull otherwise it will 
throw an error.

Note:-Procedure  is slightly different in weblogic9.2

Please ignore the previous one.I mean 8th.