This all started because I was telling a friend how bad a reputation the standard Net weapon has, in 5e... They said "How big is the net?"
I double-checked the item listing but it didn't say (actually I have two & neither say), so I started running searches online. I'm a stickler for finding any canonical points that may pertain, before I commit a game forever to some ruling, so I went on a deep dive to figure out how I'd rule on this... but I'm still not convinced that my own answer of 10ft is the only reasonable answer, nor am I certain what effects such a ruling might have.
I quickly found some numbers & got over-excited & posted this related question, asking if there's any possibility of a net hitting 4 medium creatures, which was not well framed & understandably none too well received. In all likelihood, a Net attack roll can only ever hit one creature.
But in addition to some (¿still iffy?) points about what attacks targeting a location are for, the very crux of that admittedly pretty stupid question, hung upon the size of a Net weapon being larger than 5ft×5ft! Since the item listing doesn't say that, that would obviously not a safe assumption for a player, but after quite a bit of searching & Q&A, I do think that both realism & what little text about nets exists, tend to support a GM ruling that a Net weapon is 10×10 when laid out flat.
Here's why:
A Large creature which controls a 10ft×10ft space, becomes Restrained if Hit by a net (PHB p149):
A Large or smaller creature hit by a net is restrained until it is freed. A net has no effect on creatures that are formless, or creatures that are Huge or larger.
The Creature Size Categories on PHB p191, say that Large creatures control a 10ft×10ft space, instead of 5ft×5ft for Medium & Small creatures. Absent any other data to indicate size, a net sized to cover the space of the creature it's restraining, seems like a reasonable inference to take as a starting point?
Additionally, the only sizes we do have listed for a net (including all the new rules for traps and crafting during downtime), are the nets used for the Falling Net (DMG p122) & the Net Trap (XGE p113):
When the trap is triggered, the net is released, covering a 10-foot-square area.
A net covering a 10-foot-by-10-foot area centered on the trip wire falls to the floor as a bell rings.
Those both specify a 10ft×10ft net, but those are traps which may not be representative of the dimensions for a Net weapon. As any stickler for "what is RAW" may point out, Net Traps being 10ft doesn't explicitly establish that Net weapons are 10ft. RAW doesn't cover this question (if it did, I'd have no need to ask).
So, since RAW doesn't say the dimensions, & direct comparables are lacking, perhaps it's prudent to also consider how big a standard Net weapon would realistically need to be, given the criteria that a standard Net can restrain a Large creature? Well, realistically I'd say a net needs to cover at least a 10ft area, to halt a creature that controls a 10ft area... Even a 10ft net barely reaches the floor when thrown over an average ("Medium" sized) person. Such a conclusion also fits with most media I've found depicting nets as a thrown weapon.
But the listing for the Net, simply doesn't say what size it is. We have to make an educated guess, & so far it seems like all we've got to go on, are a couple 10×10 trap listings & an item listing for the Net weapon which specifies that it can restrain a Large creature.
An entirely RAW Answer is not available, or the Question would be moot; and any Answer may earn criticism for any lapse in justification or reasoning or style etc...
I have a sinking suspicion that a 10ft Net will be problematic somehow, that I just haven't spotted yet.
Thankfully, the use cases where a net's size becomes a critical question mechanically, seem extremely limited, & the only combat option that came to mind where it would matter (Net attack as AoE), does not seem supported by RAW.
Nonetheless, for the sake of avoiding broken gameplay, & for the sake of expediting future trap crafting questions, & utilitarian usages for nets, & so on:
Is it reasonable to rule that a Net (weapon) measures 10ft×10ft, & what other gameplay implications should I consider before ruling that a Net is a given size?
Just as a note, this question originally asked for rulings that would be appropriate "supported by RAW" but an editor objected that a ruling cannot be based on RAW anywhere that raw is not fully explicit. Rather than point this out as a comment on this question & thereby grant the possibility to improve the question, their criticism of the question was inappropriately posted as an Answer (which it quite clearly is not), & my attempts at clarifying the question were then undone on the basis that, essentially, "what do the rules say" & "what ruling is supported by what text exists" are questions too different to be revised, despite no pertinent answer having been posted yet. I'm now editing it again because this is substantially the same question I originally asked & I'm just looking for some freaking opinions founded on what does already exist in the game, as opposed to deciding solely by GM fiat: Does a 10ft net actually make sense to any of you from a gameplay standpoint & either way, why?