How big does a wildflower 'meadow' have to be, to be useful?

In this case, I don't think size matters
"Undisturbed" is good for wildlife so could you sprinkle some wildflowers seeds on the patch and attract bees and butterflies to keep your frog company? They'll also be good pollinators for your crops while Mr Frog sorts out the slugs.

So as they say "Every little helps"

Go for it because every bee and pollinating insect you attract is a bonus to your plot.

Whatever you can do, helps - as above.

I have a strip, maybe a foot wide and 5' long that I have mixed flowers/bulbs in etc. I also leave the odd green manure to flower dotted about the place - sometimes it's 1 plant that's flowering amongst veg, other times it's a few.

There's "wild" and "driving other plotholders wild with anger"..

I don't plant wild flowers in our garden. On my morning walk in spring I walk along with wild flower seed in my pocket and sow them on any bare earth I can find.. Even in the town centre there are odd corners with just plain earth. It's amazing what plants crop up as a result.

(As I keep bees, I tend to use pollen/nectar bearing plants like phacelia or marigolds - large - etc)

It's called "guerilla planting". Ideal for abandonned allotment plots! :-)

Depends how you define "making a difference." On one level, all a few wild flowers will do is attract in pollinating insects from elsewhere, giving them a nectar source and a feeding place. You won't alone ensure the survival of a species or have bumble bees or butterflies nesting or hatching out on your small patch. But as somebody else said, 'Every little helps.'
The greatest mistake wild life conservation bodies have made is to mistakenly believe that you can allow wild life to survive as long as there are a few wild life reserves dotted around, like islands, here and there. Now they have realised that this doesn't work and you need habitat everywhere, so insects and mammals can move from one site to another. That is why it is important for farmers to leave field edges unsprayed and uncultivated, and councils to be far more aware of the importance of verges. Every gardener who creates a small area for wildlife is helping in this by creating another link in the chain.
If you are lucky enough to have several acres you can certainly make a huge difference and garden organically relying on predators to keep pests under control. It's far more difficult to do so when all your neighbours are spraying pesticides and weedkillers. That's why we all need to be educators and evangelists when it comes to getting everyone to think about wild life and using natural methods. If we all did, there would be a massive change for the better for our wildlife.

Dunno if this will help at all but,in my MOM pack there was a BeeMat ready seeded with a mix of :
Common Knapweed
Greater Knapweed
Golden Rod
Cosmos
Borage
Anise Hyssop
Californian Poppy
Verbina
Lady Phacelia

It measured 2metres x 1/2metre,so I guess a square metre is enough to be useful

There are 2 full sized plots on our site that have wild flowers planted round the outside edge about
0.5m x 48 metres in total and another 2 half size plots that have an area 1m x 7m and one has about hald her plot planted with wild flowers. I intend to put a couple of beds in this coming year and the Committee also plan to have a permanent wild flower meadow at one end the site.

A single flowering plant is useful, compared to no flowering plants

... you don't need to plant wild flowers. Cultivated flowers are just as good, if not better, and so are vegetables-running-to-seed
esp the brassicas and alliums

Cultivated flowers are only good if they are single flowers. Many pollinating insects find double flowers difficult to pollinate.

that's a really fantastic idea..

Bees don't deliberately pollinate though: it's simply something they do while they're getting food. And they don't need to go in the flower's front door either: bumbles can't fit into a runner bean flower, so they go round the back and puncture a hole there

But Madasfish is right that bees and other pollinators won't visit double flowers as they can't get the nectar either. We keep bees and find this to be precisely the case.