I am afraid it's not possible (and I think it's not a good idea for the Standard Shield to scan archives at all - it may have bad impact on the system performance, even if the size of the archive is limited somehow; besides, it shouldn't be necessary, because the file has to be unpacked to be activated).
You can set the
MaxFileSizeToExtract_Resident parameter in avast4.ini, but:
1. It doesn't limit the size of the archive, but rather the size of the resulting unpacked file
2. It is common for more providers, i.e. the setting limits also e.g. Web Shield and P2P/IM providers
3. The files that are bigger than the limit are not just ignored, but rather flagged as decompression bombs (so you may see it in report, for example).