Well, when you use the 'flow' option, that's what you've told tiledlayout you want -- the size/location of the figures to update dynamically when each nexttile call comes.
If you want a fixed layout, then use the tiledlayout(m,n) option where you define how many and the orientation a priori and then fill in the content when you get there.
There are some options to control the amount of whitespace between tiles to adapt the appearance but individual axes positions/sizes are not modifiable with the 'Position'properties of a normal axes object.
There is an ability to nest layouts to create nonuniformly-sized charts, see the properties documentation 'Layout' property for the details on that.
But, if you don't want the sizes changing dynamically, don't use 'flow'
