Hi Piotr,
I think the UI5 package can only be downloaded from SAP market place and not here from SCN.
If you do not need any specific component of the UI5 library which are under a special licence, you can use the openUI5 package. It can be downloaded from OpenUI5 - Download.
Regarding the optimization of the package size visit page SAPUI5 SDK - Demo Kit, but I think you know this page already 
.
Regards,
Florian