How would you deal with this?

[jukejuke]

Posting for advice/opinion as it's making me feel ungrateful and I just want to know what others would do.
Popped round to my parents this evening and they mentioned they were on the way to pick up my sons Christmas present. A bike off eBay.
My dad showed me the picture and I was polite and said that's nice (although I really wish they had spoken to me first, it looks huge, my son already has a bike he loves and my daughters recently got one too so seems a bit strange to choose that and then she will be expecting something similar?)

Kids are 5 and 3 and the bike looks nice enough. But I did really tactfully try and say "I wish you'd have let me know, I don't think he's going to be able to use that for a very long time I'd have gone for a smaller size"
My dad knows everything there is to know about everything and regularly undermines my parenting choices or any choices I make actually, he always knows best so tactful but assertive was what I was going for but he kept saying no he will grow into it, I'm getting this blah blah.

My son was only just 5 a few months ago. He has a 12" bike that fits great as he's the shortest in his class. He's good on it but fearless so comes off quite a lot. The bike my dad is buying is 20" which is a hell of a jump for a fearless 5yo from a 12" bike with tiny legs.
I tried to be as nice as I could and say I'd rather tell you now than you waste your money but he kept telling me I didn't know what I was on about and that it would keep for him, not even in a nice way more in ah ah I can't be bothered I'm just getting this way. He was completely dismissive of me and just plain rude telling me I didn't know what I was on about but I do.

I know what will happen, he will buy the bike without talking to me, without listening to my concerns. I'll watch my son (who to be fair doesn't need anything from them for Christmas id be happier if he sat and got to know him instead) open the bike and he dissapointed he can't use it as it's huge, and then it will go rusty in their garden (we have no room for anymore bikes here) while he shouts at us it's in the way when we go round even though we didn't want it anyway.

I feel really ungrateful for thinking all of this and being miffed but surely you would speak to the parent before buying something huge like a bike for a child you don't even bother with? I just feel sad for Ds

DS is 7, tall for his age, and I'm pretty sure his bike is smaller than 20 inches. I think you'll have to store it for a while.

Sounds like the gps want to tick the present box.

I suggest you happily receive the gift, explain to DS it is of no use to him for a while, sell it and buy something he will enjoy.

Question is would the gps find out?

You’ve tried to tell him and he’s not listening. Sell the bike, either buy something of DS’s choosing or keep the money until there’s something he wants. When your father asks where it is, be truthful, ‘I sold it on because, but I told you BEFORE you bought it, it was miles too big for DS and I have nowhere to store it, you knew that BEFORE buying it’.

If he keeps on, just tell him you’re not doing discussing it. Find your inner adult.

Not commenting on your dad's approach, but fundamentally he's right. No young boy ever feels disappointed to get a big bike. He'll grow into it.

Great minds think alike sunset

disquieted and where is she supposed to keep it for 3+ years until he ‘grows into it’ and WHY should she have to?

@Disquieted1 he will be disappointed to get something he's not allowed to ride probably for another 3 or 4 years and too young to understand why?
I don't want him (knowing his nature) trying to use it and hurting himself either

Even when he outgrows the bike he has now he will likely need either a 14, 16 or 18 inch before this is even close to suitable

Why ask for other opinions?

And also for context if it helps. He got him a go cart last year that was battered and had only 3 wheels. Instead of not bothering at all or finding another wheel he used a screw and a disc of wood.

That was pretty disappointing for Ds but we were all expected to make a big deal. Yet when Ds has a sports match or a school achievement he won't even fake an interest in it. Feeling deflated

Honestly? I would just smile and say thank you. It's not a big deal.
My grandparents got me all sorts of strange/unsuitable presents when I was young. It was a useful lesson in being polite even if secretly disappointed.