To change logo (On Zimbra 8.6 and onwards)

zmprov md mail.dscsc.mil.bd zimbraSkinLogoLoginBanner https://yourdomain.com/zimbra/MyLoginBanner.png
zmprov md mail.dscsc.mil.bd zimbraSkinLogoAppBanner https://yourdomain.com/zimbra/MyAppBanner.png

To change logo height:

changed hhe contents of file /opt/zimbra/jetty/webapps/zimbra/skins/_base/base3/skin.properties

...
90 LoginH1 = margin:0 0 30px 30px;
91 LoginAppName = display:none;
92 LoginLabelColor = @TxtC@
93 LoginButton = @ButtonShadowDefault@ border:1px solid @ButtonBorderColor@;
94 LoginBannerImg = @LogoImgDir@/LoginBanner.png?v=@jsVersion@

//95 LoginBanner = @img(, LoginBannerImg, 450px, 36px)@ //old
95 LoginBanner = @img(, LoginBannerImg, 450px, 90px)@ //new

96 LoginFooterColor = color:@darken(AppC,55)@;
97 LoginErrorPanel = background-color:#FF9; padding:5px; @roundCorners(8px)@
...

Apply it-

zmmailboxdctl restart