Birding comes third to hiking and photography for me, and I find my old 8x30 Bins are too heavy and fiddly to use (especially with specs).
I want some new general-purpose ones I will actually carry (and with better Eye Relief).
My first question (having checked the effects, but before considering brands) is whether to choose Objective size 25 or 32 - knowing the bigger will be brighter and easier to use BUT I know I won't bother to carry anything big and heavy regularly.
Then whether to choose magnification 8x or 10x - knowing the first's wider view will be steadier and quicker to find subjects, but 10x will show me more satisfying detail (and be another bonus over my old 8x30).
If only Zooms really worked.
With a budget of under £100, what sizes should I choose ?
10x will NOT show you more satisfying detail with an objective that small, the tiny exit pupil and hand shake will negate any increase in magnification. Now, it would be one thing if you were willing to spend £1500 on a Leica or Zeiss 10x32 but for £100.... not so much. Chasing extra magnification is often a fool's errand.
It's very hard to find a decent compact below 30mm objective size... I would try to find the lightest, most compact 8x32 I could find. There may be some decent 8x28's around also...
you can get some fairly good compact bins that will fit in a jacket pocket for around that price. I'd go for 8x magnification myself that way your image should still be reasonably sharp.