Answer: Unfortunately I am not able to think of anything that would be evergreen and offer year round flowers in that very small size range. But, here are some suggestions to consider.
You might consider a dwarf shrub with long season appeal such as a smaller growing Nandina (Woods Dwarf grows to 18 inches tall and wide) or possibly one of the lower growing, spreading azaleas. Another possibility might be Leucothoe (Leucothoe fontanesiana 'Rainbow' can be kept at 18") or possibly a Himalayan Sarcococca (it too would require trimming to stay that narrow.) Another possibility might be to use a small maturing dwarf conifer such as Chamaecyparis pisifera 'Baby Blue' which does not bloom but has a lovely blue foliage color year round and grows slowly to about two and a half feet tall and wide. Or, if you truly want flowers from spring through fall your better choice would probably be an annual flower such as a mid size to larger marigold or certain petunias such as the Wave series. These would be replaced each spring.
Your specific choice would depend on the growing conditions (sun, shade, moisture, soil type, wind exposure) in the spot where you want to plant. I would suggest you work with your local professionally trained nursery staff to analyze the growing conditions where you want to plant and identify plants in that size range that would thrive there, then finally select something you like from among those.
Q&A Library Searching Tips