So you are saying mempool have sizes and they are different from each other or this is determined by the node you're broadcasting on.
Each mempool can set different memory pool and they can set their max mempool size, mempool expiration.
Like this
https://jlopp.github.io/bitcoin-core-config-generator/This example setting has max mempool size is 500 megabytes, mempool expiration (hours) is 336 = 14 days.
mempoolexpiry=336 (hours)
mempoolminfee =0.00001000
mempoolminfee is used to set min fee rate and transactions with lower fee rate than this will be dropped from a mempool but it can be kept in other mempools that have min fee rate is lower than this.
https://chainquery.com/bitcoin-cli/getmempoolinfogetmempoolinfo
Returns details on the active state of the TX memory pool.
Result:
{ (json object)
"loaded" : true|false, (boolean) True if the mempool is fully loaded
"size" : n, (numeric) Current tx count
"bytes" : n, (numeric) Sum of all virtual transaction sizes as defined in BIP 141. Differs from actual serialized size because witness data is discounted
"usage" : n, (numeric) Total memory usage for the mempool
"total_fee" : n, (numeric) Total fees for the mempool in BTC, ignoring modified fees through prioritisetransaction
"maxmempool" : n, (numeric) Maximum memory usage for the mempool
"mempoolminfee" : n, (numeric) Minimum fee rate in BTC/kvB for tx to be accepted. Is the maximum of minrelaytxfee and minimum mempool fee
"minrelaytxfee" : n, (numeric) Current minimum relay fee for transactions
"incrementalrelayfee" : n, (numeric) minimum fee rate increment for mempool limiting or replacement in BTC/kvB
"unbroadcastcount" : n, (numeric) Current number of transactions that haven't passed initial broadcast yet
"fullrbf" : true|false (boolean) True if the mempool accepts RBF without replaceability signaling inspection
}
Examples:
> bitcoin-cli getmempoolinfo
> curl --user myusername --data-binary '{"jsonrpc": "1.0", "id": "curltest", "method": "getmempoolinfo", "params": []}' -H 'content-type: text/plain;'
http://127.0.0.1:8332/