estimation of program capacity

Does anybody know any methods or references to estimate the memory capacity which my logic needs that? I haven't chosen the brand of PLC yet, and I need a good way for calculation of needed capacity in CPU memory.
Thanks

Memory consumption varies between different brands and models, making it very difficult to answer your question.

Unless you know in advance that you will have exceptional memory needs, make your choice based on I/O count. My experience has been that the PLC will have sufficient memory to support the number of I/O modules allowed by the model you choose.

If you choose an AB Logix PLC you can check the memory usage in the controller properties dialog (on the memory tab I think).

As someone who recently carried out some work for a customer on an existing control system I was relieved when they were happy to pay extra for a plc upgrade when I found that the plc had became full while trying to install all of my additional software. Are there any general guidelines or calculations where you can work out roughly how much memory a certain number or rungs or various instruction types will use up?

what others have already described is the way to go, but ultimately it is dependent on how many rungs, instructions, memory spaces you use, to not that some instructions take up more memory than others. As Steve has already mentioned account for the number of I/O modules you need plus leaving room for spare IO is good practice. if you can tell us what kind of application you are doing, and roughly how much IO you need, then we can probably give you a better answer

It's very hard to estimate without knowing the project. Most of the memory in my controller is used up by a large recipe data array. One thing that always uses a lot of memory though is a servo axis.