I photographed this flower last Saturday (central Spain). I'm pretty sure it is a field forget-me-not.
I found a website which describes the field forget-me-not has having flowers from 2 to 4 mm in size. (http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artjun05/bjforgetmenot.html)
I measured this flower by counting pixels and it is 1.02 mm. Is the website wrong, is my identification wrong or is this an abnormally small specimen?
I guess than the website is right, your ID is right (this is clearly a Myosotis species) and that this is probably a normal specimen, but the measures using the pixels maybe gave you a not exact result. If you don't remember it very "microscopic" in the moment of the photograph, probably is a bit bigger. I would say about 1,6 to 2 mm as the minimum, based on the flowers that I've found in the field.
Well, I've re-checked my camera data and I stand by my measurement. Yesterday I took a photo of a field forget-me-not close to the original flower with the same camera, same lens, same focal length etc, using a 2 Euro piece to enable me to measure its size. It was 2 mm. Below is a crop of that photo which I've attempted to crop to the same extent as my orignal, for a direct comaprison.
In fact you can see some flower buds in the two photos are the same size but it's just the flowers at the top of the original photo are very small.
there are a couple of things I'd like to ask.
Firstly was the plant with the smaller flowers looking stressed? The leaves and stems look like they are flushed red with anthocyanin. Were all the plants, including your second specimen, similarily coloured?
Another point is that the first plant looks like it is getting to the end of it's flowering season - the open flowers have few buds to come after them, whereas your second specimen still seems to have several more flower buds still to open. Was this the case or is it just the angle from which the initial image is taken?
If you are confident in your ident as M. arvensis and the ripe nutlets confirm your id, then it is probably a simple case of phenotypic plasticity at work. It may be - and I hasten to add there is no way to check this hypothesis for this specimen! - that the first plant is getting to the end of it's growing season, it has flowered well and set seed adequately. The climatic conditions have been putting the plant under increasing levels of stress - lack of water, UV levels increasing and causing high light stress, etc. All of which make photosynthesis more difficult.
Under those conditions the plant will have a limited ability to fuel the necessary metabolic processes and to invest in the essential process of seed production and maturation. Any resources available will be invested in seed development and maturation the development of flowers will be secondary. But as flower development still continues, the size of the flowers - the entire perianth of calyx and petals - will be smaller than "normal", aspects such as amount of pollen produced will be reduced too. In species with multiple seeds per carpel fewer ovules will be produced.
Also do bear in mind that the flower range given in a flora is for those flowers falling within the 95% confidence of the bell curve of normal distribution, there will always be outliers, sometimes extreme ones - just think of the variation in height found within humans... Or the difference in size between a Plantago coronopus growing in a lush sheltered postion and one growing in a crack on rocks exposed to sea spray and trampling. Both reproductively active, but with a huge variation in leaf and inflorescence size and productivity.
I hope that helps and does not confuse! Basically if you are happy with your id then that is what it is... Flower or leaf/rosette size are never definitive guides to species as they are all influenced by environment acting upon genetics - phenotypic plasticity. Flower, leaf and seed characters and arrangement are developmentally locked by the genetics of the plant and so much better characters to determine species by.