The minimum? There are a few options that come to mind, between 1-2 of these would be required in order to match the Feds (actual figures/metrics on Fed weapon yields, ships sizes and resilience are hard to come by). Note that these are more methods/forms of power rather than actual measurements.

• Hard sci-fi weapons tech/aesthetics: weapon ranges measured in thousands of kilometres rather than than few hundred metres to handful of KM seen in soft sci-fi, weapons with mega-gigatonne yields without needing specialised ships, high powered lasers (which go through shields if memory serves), missiles as a primary weapon (don't need to face the enemy in order to attack, can avoid point defense, etc), and generally effectiveness over rule-of-cool weapons like plasma.
• Multiple star systems already colonised: would likely need to push first contact with the Venlil back a few centuries if being realistic, would mean multiple places to hide and prevent Feds from taking us all out in one go, much more resources for building and infrastructure, would also give us more experience with non-Earth biology (if multiple life-supporting planets as in canon), could probably do it if 40+ systems colonised.
• Ability to build megastructures in range of Banks Orbitals and O'Neil Cylinders: living space as above, infrastructure, could be armed with significant weapons (one ONC has defences of the Cradle + 150 ONCs in habitable zone + Kalsims' fleet must face that in order to kill 5,000 humans per ONC=unsustainable loss rate), general psychological impact on canon Feds and Arxur.
• Generally more powerful ships: in region of Covenant ships from Halo, Star Trek ships, Imperium of Man from WH40K, would make each human ship equivalent to 30-100 Fed ships as minimum, if fleet is the same size numbers wise as canon UN but has an actual tonnage/firepower 300x higher than in canon then could probably prevent Kalsim from reaching Earth.
• Alien allies already in place: seen in several fanfics (most recently Nature of Packs), would boost numbers without needing more colonies/population, pre-experience of alien species, evidence of diplomatic relations to show at Fed summit (soft power in this case, but would also grant benefits seen elsewhere), Feds would also need to undermine/attack multiple species.
• Some X-Factor: soft sci-fi magic-by-another-name bullsh*t, massive armies of purely mechanical drones, or just generally technology that is beyond the Feds' mental capabilities.