As thrash says, the problem is that the planet moves faster at perihelion, so the summer will always be shorter than the winter. If you want the 'seasons' to be more similar in length, you'd need to have a smaller eccentricity, but that also makes the difference between the 'seasons' less extreme.
Earth's blackbody temperature (not including atmosphere and albedo) without any eccentricity at all is a flat 278K (at 1 AU).
The variation caused by Earth's current eccentricity of 0.0167 is +2.36/-2.30K (orbital distance varies between 0.983 and 1.017 AU)
With an eccentricity of 0.05 that would be +7.24/-6.72K (orbital distance varies between 0.95 and 1.05 AU)
With an eccentricity of 0.10 that would be +15.07/-12.97K (orbital distance varies between 0.90 and 1.10 AU)
With an eccentricity of 0.20 that would be +32.89/-24.28K (orbital distance varies between 0.8 and 1.20 AU)
With an eccentricity of 0.30 that would be +54.40/-34.26K (orbital distance varies between 0.7 and 1.30 AU)
Keep in mind that the summer and winter variations we see are caused mostly by our tilt though - in northern hemisphere summer we're actually furthest from the sun, and in northern hemisphere winter we're actually closest. (see http://spaceweather.com/glossary/aphelion.html
). But if we had our tilt and even an eccentricity of 0.05 we would REALLY notice the extremes!
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