The size of the application shud be kept to minimum , may be under 300K-400K as some of the WAP gateways might block the big application.But i once saw a sis file of 1.6 MB also .Thats a game for P900.
U cannot seperate the SIS into several parts .
"In addition, is 'IMEI checking' the only possible secure method to ensure each copy of the application is legal (if the application is installed by file transfer from PC)."
I think yes specially when u talk of transferring from PC.
Compressing the application does not help reduce SIS size since SIS files are already compressed.
Generally, the memory available on a device limits SIS sizes. The SIS file has to be stored on the device. The installer needs some working memory. The installed application takes up some memory. Of course, the transport media (for example WAP/WSP) can pose additional constraints on SIS size.
Last year I was toying around with bundling multimedia content in an application SIS. A couple of 3650 devices needed to be formatted after trying to install a 800kB SIS file on them... the applicaiton could not be completely installed, it could not be removed and almost all memory was exhausted.
I don't think there is a limit to size of .sis file. Just available memory on the phone (remember you need memory for SIS file and at the same time memory for the installed application). I was able to install sis file bigger than 1MB on N3650 and SX1 without any problems...