It's spring for you now right? Do you know what variety they are? Strawberries can be divided into essentially three types - What we call "June bearers" that produces lots of strawberries all at once in late spring/early summer for period of about 2~3 weeks, "Day Neutrals" that produce a flush of smaller number of berries around the same time as June bearers, then some more much smaller numbers every month, and "Everberers" which though I've never had them myself I understand to basically grow two large crops in spring and in fall.
If yours has been fruiting for nearly 6 weeks, I would think it's normal for the berry size to start diminishing.
You don't mean the plants themselves are getting smaller, do you? I believe what you are talking about is a Strawberry Jar (It is a kind of a container -- as opposed to being grown directly in the ground
). Sometimes, those jars are not big enough. Sometimes the issue is water/moisture distribution in the soil and they dry out easily. The usual recommendation is to bury a perforated PVC or wire tube filled with pea gravel as a central column and pour water into the tube to water. ...but you have to do this at planting time.
Any time you're growing in a container of any kind, available soil nutrients are washed away as well as used up, so you need to fertilize once a month, or else every two weeks with diluted fertilizer. AACT -- Actively Aerated Compost Tea is one example and is described in a long sticky thread at the top of the Compost Forum.