I have seen sizing information for servers, but how high do you judge the maintenance (server, software, plugins) and administrative overhead (besides creating new users and managing groupmembership) for a 250 userbase installation of JIRA and confluence ? (E.g. Compared to sharepoing or alfresco sites)
You can't really "size" the effort involved without referring to the complexity of what you're going to do. If you're going to use the applications pretty much off-the-shelf and not fiddle around with themes, non-atlassian integrations, workflows, addons, fields, and and and, then the maintenance is quite low - you'll need to think about a part-time admin who can do the odd upgrade. But if you are power-users who are going to heavily customise lots of it, then you'll need more. I've looked after several 200-ish user sites. Some needed me for a day a month, others near-on full time.
In addition to the very good answer from @Nic Brough [Adaptavist], In case you are wondering for best practices in terms of tuning Confluence instances (depending on the instance size, you may have to perform that), here are very good starting points:
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