Merge PR cosmos#2375: Add an example for the size of a transaction

* Add an example for the size of a transaction

This shows that the size of a send tx from 1 input to 1 output is 173 bytes
This is good information to have, though we need to find the correct place
to put it.

* fix lint

cmd/gaia/app/benchmarks/txsize_test.go

@@ -0,0 +1,35 @@
+package app
+
+import (
+	"fmt"
+
+	"github.com/cosmos/cosmos-sdk/cmd/gaia/app"
+	sdk "github.com/cosmos/cosmos-sdk/types"
+	"github.com/cosmos/cosmos-sdk/x/auth"
+	"github.com/cosmos/cosmos-sdk/x/bank"
+	"github.com/tendermint/tendermint/crypto/secp256k1"
+)
+
+// This will fail half the time with the second output being 173
+// This is due to secp256k1 signatures not being constant size.
+// This will be resolved when updating to tendermint v0.24.0
+// nolint: vet
+func ExampleTxSendSize() {
+	cdc := app.MakeCodec()
+	priv1 := secp256k1.GenPrivKeySecp256k1([]byte{0})
+	addr1 := sdk.AccAddress(priv1.PubKey().Address())
+	priv2 := secp256k1.GenPrivKeySecp256k1([]byte{1})
+	addr2 := sdk.AccAddress(priv2.PubKey().Address())
+	coins := []sdk.Coin{sdk.NewCoin("denom", sdk.NewInt(10))}
+	msg1 := bank.MsgSend{
+		Inputs:  []bank.Input{bank.NewInput(addr1, coins)},
+		Outputs: []bank.Output{bank.NewOutput(addr2, coins)},
+	}
+	sig, _ := priv1.Sign(msg1.GetSignBytes())
+	sigs := []auth.StdSignature{auth.StdSignature{nil, sig, 0, 0}}
+	tx := auth.NewStdTx([]sdk.Msg{msg1}, auth.NewStdFee(0, coins...), sigs, "")
+	fmt.Println(len(cdc.MustMarshalBinaryBare([]sdk.Msg{msg1})))
+	fmt.Println(len(cdc.MustMarshalBinaryBare(tx)))
+	// output: 80
+	// 173
+}