It is not right to call the xenophyophore that is on the last row, and which can have a size of up to 20 cm as an "amoeba".
Only the next row above it, with Pelomyxa, is indeed an amoeba and one that is very frequently encountered and which usually has sizes not much less than 1 millimeter and sometimes it can reach a size of a few mm.
The true amoebas are much more closely related to humans, than to xenophyophores (giant marine unicellular living beings) or to plants.
Besides the true amoebas there are also a few other kinds of unicellular eukaryotes with shape-shifting cells, e.g. foraminifera, radiolarians and others, but already in the first half of the 19th century it was recognized that those other groups change their shapes in a different way than the amoebas, so they were classified separately, even if the term "amoeboid cell" has always been used about any cell with variable shape.
The true amoebas are related to the group formed by animals and fungi, and there are some amoebas that have a simple form of multicellularity, so it is likely that some of the mechanisms needed for the evolution of multicellularity have been inherited from a common ancestor of animals, fungi and amoebae.
The multicellular or multinucleate amoebae that belong to Myxomycetes (one of the kinds of slime moulds) can reach much bigger sizes, e.g. a diameter of up to 1 meter, because they do not have the size limitation that exists for simple unicellular eukaryotes.